49

THE OF AMMONITES.* A DEMONSTRATION GIVEN AT THE BRITISH MUSEUM (NATURAL HISTORY).

SATURDAY, 8TH FEBRUARY, 1919.

By W. D. LANG, Sc.D., F.G.S. (J:}' TI.'l'missioH (If the Trustees of tilt British Museum.)

I.-INTRODUCTORY. HE Evolution of Ammonites may be described as a phrase with two faces looking in opposite directions. From theTview-point of a primitive Ammonite, one face looks back to processes whereby Ammonites have been produced from organisms which Were not Ammonites; the other looks forward to the developmental history of Ammonites as such, as they diverge along multitudes of lineages, attain importance. then dominance among marine invertebrates, and finally become extinct. The subject is manifestly immense; and on such an occasion as this only hints can be given here and there of the apparent method of evolution, and perhaps a few suggestions, little better than guesses, as to some proximate causes. It would, perhaps, be true to say that a large treatise on Ammonites could do no more than this, and that all that can be hoped for now is to indicate some of the directions such a treatise would pursue; or to create an atmosphere, or attitude of mind. in which the methods of Ammonite evolution can better be understood. It is unfortunate that Ammonites suffer from their termino­ logy. To the uninitiated, the talk and writings of Ammonite specialists tend to appear to be an unnecessary and unintelligible jargon; and the amateur is discouraged from the study of what appears to him an attractive subject demoralised into a mystery by a few keen terminologists. It is unnecessary to say that this is a mistaken view, but it is one which appears to come very naturally to the amateur. Perhaps a demonstra­ tion given by an amateur (as far as Ammonites are concerned) to amateurs may help this difficulty. It is a fact that the difficulties of Ammonite-study confront the beginner at once. "There is no royal road to sutures," was once said by an Ammonite specialist; and this dictum applies with equal force to other Ammonite matters. There is not even the pleasant *Most of the facts and many of the opinions given below, as well as a wealth of additional information, may be found in C. R. Eastman's translation of L. von Zittel's Text-book of r akvcntotogy, (1900) and inA Guide to tne Fossil Invertebrate Animals in the Department of Gcoloi!Y ami Palaotttoloey in the British. Museum (Nat. His! .}, 1907.

PROC. GEOL. Assoc., VOL. XXX., PART 2., 1919. 4 50 W. D lANG ON by-path for beginners that so man y subjects afford before the foot-hills of difficulty are reached . On th e contrary, the difficulties presented by Ammonites become mountain ous immedi ately the asthetic is ex changed for the scienti fic view. Terminology is a necessary aid brought forwar d to help to surmount these di fficulti es, and, if thus accepted, is not so troublesome to mast er. And if the terminology is crabbed , it is because the study of Ammonites is complex. Hence such terms as pro toconch, oxycone, homceomorph, and tachygenesis, ceasing to terrify, may begin to help ; and from being bad masters may become good servants. Perhaps a greater dr awback to Ammonite- study than a crabbed ter min ology, is th e extent and diffusion of Ammonite literature. But this mainly concerns their syste matics, and our present theme is evolution . Now a fundamental prinriple that has hitherto guided investigat ion into Amm onit e evoluti on is the so-called law of Recapitulation. Some discredit has been thrown upon this principle by workers in Recent zoology, because they have chiefly sought for it s expression in embryonic stages . But the immediate environment of an embryo is so special, that the ancest ral traits expected to be visible in its development are wont to be masked by other features caused by to its peculiar surroundings. In a fossil, how­ ever, we cannot ord inarily have direct knowledge of any growth­ stage that preceded the shell. And , though an embryonic shell is supposed to have been formed , that is, a shell present within the egg ; yet it is on recapitulation exhibited in post­ em bryonic growth-stages tha t the palreontologist chiefly relies for eluci dat ion of an Ammonite's evolut ion . For instance, an Ammonite from the Dorset Lias, which is generally known as lY1icroceras densinodus,* shows at its centre the so-called embryonic shell, or protoconch , followed by about lour whorls which have no ornament: then follow from two to th ree whorls with ri bbed or costate ornament; an d, finally, a stage in which the ribs acquire tubercles near the periphery . Consequent ly , it is supposed that the ancestral form had a plain shell, and that, during its evolution, the stock, or lineage from which .1.1'1. densinodus arose acquired, first a ribbed , and then a tubercula te ornament. This succession of plain, costate and tubcrculate stages in Ammonite ornament is shown in nu merous group s quite unconnected with M. densin odus and with each other (for instance, in the Liparo­ ceratid lineage discussed further on), and illustrat es a second principle fundament ally affecting Ammonite evolutio n, namely . that of , whereb y t he same general sequence of

"Tbe particul ar spec ime n exhibited has been labelled by ~Ir. S. S . Buckman ,;\/jrf"oceras obsoletn m Blake (non Simpson) sp. \Vhile gratefully accepting Mr. Buckman's determin ation . J have, for the sake of am at eurs, used at least the trivial nam e by w hich th is form and probably other similar, but specifically different for ms, from Charm ou th are generall y known . THE EVOLUTION OF AMMONITES. 5r structures is repeated again and again in different lineages springing from one ancestral form, so that it is possible to some extent to predict the after-history of a given primitive species. Bearing in mind, then, these two principles, namely, that of Recapitulation, following which the individual during its development repeats the history of the race; and Orthogenesis, whereby it may be predicted that the lineages diverging from an ancestral form will severally run through similar consecutive stages, let us first of all briefly consider the evolution which in latest Palseozoic times culminated in the Ammonites.

Il.-EvOLUTION LEADING UP TO THE AM,\WNITE. The nearest approach in appearance to an Ammonite among Recent organisms is the Pearly Nautilus. There are manifest differences between an Ammonite-shell and the shell of Nautilus (fig. r}: notably, in Nautilus, the simply-curved suture, the median siphuncle with backwardly-projecting septal

'I b

Fig. I. A Nautilus :.HHl an Ammonite. a is a plaster cast of the inside of the shell of the Recent "';aulitus ponipilins, and may be compared directly with the _si m ilar iutr-rnal cast (h) of the Ammonite. .c, sururc : ni. mark left by muscle of attachment; I. !Ines left by this mark in its previous positions. The slphu.llcle WIth.ItS se~ta! necks are not shown in these views, hut the position of t~Je slphu~de In (b) IS nH!Icate(l by the peripheral keel. (From British Museum c.mdc to Fossil Invertebrate Anirnals.) necks, the comparatively unornamentcd shell, and a ventral bay or sinus at the aperture of the shell; and, in an Ammonite, the. cOJ.TIplex suture, the perip~eral siphuncle with forwardly­ proJect~ng necks, a shell often highly ornamented, and a ventral projection at the aperture of the shell. But these differences are not great enough to preclude the probable affinitv of the tw.o shells; a.nd pakeonotology not only confirms their 'relation­ ~ll1p, but ~luCldate.s Its nature. Na tilus among Recent molluscs IS a peculiar and Isolated form of the order Cephalopoda. The 52 W . D LANG ON other Recent Cephalopods are the Cuttle-fish, Squid, Oct opus and their near allies. (The shell of the so-called 'Paper Naut ilus ' is merely a brood-cha mber for the eggs of an Octopod, and ha s no homologies with the shells of Nautilus and the Ammonites .) Those Cephalopods havin g shells built on lines similar to that of N autilus compose the group N autiloidea ; while Cuttles, Squids, Octopods and such-like are the B elem­ uoidea ; and the A mm onoidea comprise the Ammonites and their near relatio ns . Now the earl iest growth-stage manifested in Ammonites is the so-called embryonic shell, or P rotoconch (fig. 2), It is reasonable, therefore, when investi gating their mutual relation­ ship, to compare the protoconchs of ammonoids and N autilus.

F ig. 2 (on left). A much-en largerl secti onal view of t he apex of th e shell of th e Ammoni te P ali oplruroceras spina /11m, sho wing th e large globular pro to conch (p) with t he blind end of th e siphuncle pro ject ing into it. The posit ion of th e siphun cle is indicated and it is seen rather rap idly edging to t he periphery ' of th e whorl ; but t he sept al necks and th e di scon tinuit y of t he se pta where they meet the siphuncle are not shown , (Adapted, after Branco, fro m Zittel's Text­ book of Palzeontol ogv}. F i g. 3 (on right ). A mu ch-enlarged sectiona l view of t he apex of t he shell of Nautilus pompilills, showing the absenc e of a protoconch. The firs t three chambers are shown complete \~'j t h a median si phuncle and backw ardly-pro­ jec tmg se ptal nac ks . aniy t he m nerrnost parts of chambers four to nine are s hown, and are represented as hav ing thc greater par t of each bro ken away. (Ad ap t ed, after Branco , from Zittel's Tex t-book of Palaontology.) But, when we come t o th e apex of a Nautilus-shell, there is no protoconch visible, only a scar, showing where a presumed protoconch was once attached (fig. 3). If the Ammonites differ from Nautilus at so early an age, it is likely that their relationship is not so very close. And the next step in our quest is to follow Nautilus back through the ages until an an­ cestral form is reuched from which the Ammonites also can be derived. Of t he. nautiloids, Ilautilu« is the only surviving genus. THE EVOLUTION OF :\:MMONITES. 53 But in Tertiary times there were also, along with Nautilus (or genera very closely allied to and generally referred to Nautilus), two other genera, namely Aturia and Hercoglossa, superficially resembling Nautilus, but with their sutural curves more pro­ nounced and somewhat angular. These three genera can be traced back to the Cretaceous. In the Jurassic we still have Nautilus (sensu lato) and Hercoglossa, and in addition Pscudonautilus allied to Hercoglossa. But when the Palreozoic is reached, besides Nautilus, nautiloids are found in great variety of lorm and sometimes in great abundance. Nautilus and the other nautiloid genera hitherto named have very tightly­ coiled spiral shells, and one whorl to some extent overlaps the preceding. Such shells are called involute, and involute nau­ tiloid shells are nautilicones. But many palseozoic nautiloids are so loosely coiled (e1Jolute) that the whorls only just touch, or are not quite in contact; these are gyrocones, and typified by the genus Gyroceras. Others (Cvrtoceras) are but curved, and are oyrtocones (fig. 4). Finally, orthocones are straight nautiloids like Orthoceras (fig. 5). Obviously here is an evolution starting with an orthocone and proceeding through cyrtocone and gyrocone to nautilicone. And already we are prepared for this evolution not having occurred hut once, but, on the contrary, being the normal sequence for every nautiloid lineage. Incidentally a third evolntionary principle is illustrated among the multitudinous developments of these energetic nautiloids. Any character, such as the coiling of the shell (just considered), runs a pro­ gressive course (anagemst's)* along a given lineage; reaches a maximum (acme); and often shows a retrogressive stage (catagenesist) before the lineage dies out. But different accom­ panying characters evolve independently, with the result that, in anyone lineage, their acmee do not necessarily coincide. An outcome of this principle is seen if the nautiloids are viewed as a whole, when it appears that" the acme of siphuncular differen­ tiation occurred in the Ordovician, of general morphic diversity in the Silurian, of ornamentation in the Devonian, and of sutures in the Trias."t It is somewhat disappointing in our quest for a nautiloid from which the ammonoids can he derived, to find that even when we get back to the nautiloid orthocone Orthoceras, there is no good protoconch. There is either a scar, or a solid plug, or other " shrunken remnant "§ of a protoconch. Zittel.] however, describes a nautiloid orthocone which he calls Proiobactrites as the direct ancestor of the "ammonoid orthocone " Bactrites • A. Hyatt, 1889, Genesis of tile Arictidiv. Smithsonian Contributions to Ksunrledec, No. 673. p. 71. t A. Hyatt. ,889. op:

F ig. 4. A cyrtocone- P IO!o/, l1 m glll occm s murchisoni (Barr.mdo}. Siluri an Boh em ia. (Aft er Zittel.)

h<>J,. ·chamb" partl )· {lIlt" (}y ,nal, i%

,hcll u,,11 $"1,1111" .tf>lal lire" •.-phullcl,

.sui:"" SUit lotftrr""r matri x 1i11111~ cham­ bcrs

Oll' IWIl ;,l ti o"/sic(, (1./ 51,(./-". .1 I

apc,; a b Fig. 5.. Two orthocones . (a) Piloceras, a form with a com plex siphuncle; and. (b) Orthoceras with a simple siphuncle. In Piloceras the dotted lines are re­ constructed; sw, shell-wall ; s, chambers divided by septa wh ose neck s form the wall (1£1) of the wide neck-tube ; sc, space occupied by visceral cone, the har dened skin of which forms the sh eath (sh) ; r , rema ins of similar sh eaths ; e, endosiphon , I n the lower part of fig. (b Or /hacems) t he shell-wa ll is preserv ed ; then it is partly removed , showing the filling of rock or mat rix ; higher up thi s is cu t thr ough, and at t he t op it is removed , showing t he ot her side of th e sh ell. ( From Bri tish, Museum Guide to Foss il I nverteb rate Anima ls.) THE EVOLUTIO:-< OF AMMONITES. 55 (to he further considered). But Bactrites has a calcareous proto­ conch. It is implied, the ref01 e, though Zittel does not expli­ citly state it, that Protobactrites also had a protoconch. It is possible, however, that primitive nautiloids, Protobacirites among them, had a chitinous protoconch which perished during fossilization of the calcareous shell, and that in their ammonoid descendants calcareous matter invaded the protoconch, which thus became capahle of preservation as a fossil. Bactrites (fig. 6) was spoken of as an " ammonoid orthocone," We haw seen that it has a straight shell resembling Orthoceras, and a calcareous protoconch which was almost certainly present in a chitinous, if not in a calcified form in its nautiloid ancestors. In what respects does Bactrites differ from Orthoceras

Fir;. 6. Section through Bacirites, showing- the excentric posrtron of the siphuncle and the backwardly-projecting septal-necks. The apex is represented as having been broken away. that it should be called an ammonoid rather than a nautiloid? We have seen that the chief differences observable between a Nautilus-shell and an Ammonite in particular, as between a nautiloid and an ammonoid in general, are, in N au/flus, a sim ply­ curved suture, a median siphuncle with backwardly-projecting septal necks, a comparatively unomamented shell, and a ventral bay or sinus at the aperture of the shell; and, in the Ammonite, the complex suture, the peripheral (ventral) siphuncle with forwardly-projecting septal necks, a shell often highly orna­ mented, and a ventral projection at the aperture. Now, of these ammonoid characters, Bactrites shows only the peripheral siphuncle. The other characters are gradually acquired during ammonoid evolution. Thus, the majority of characters in Bactrites are nautiloid. But in nature there is no respect for 56 W. D. LANG ON mere majority, and the peripheral siphuncle, combined (£lith the after-history of its group, proclaims Bactrites an ammonoid beyond possibility of question. Bactrites is thus a primary ammonoid radical--an ammonoid orthocone or bactriticone. 'vVe are prepared for a coiling of the ammonoid shell during further evolution similar to that occurring in nautiloids. So, along many lineages, there are loosely-coiled forms called mimocones, named from the genus Mimoceras (now called Gyroceratites) , and tightly-coiled ammoniticones, so called because all typical Ammonites possess them. Further developments (other than catagenetic) of shell-shape taking place in ammoniticones mainly concern the cross-section of the whorl; such are cadicones, oxycones, etc.; but they need not be considered at this stage. We have now traced the primary ammonoid from a nautiloid ancestry, and have yet to attain to the Ammonite. It would be wearisome to describe in detail the various main

Fig. 7. Diagramatic median section through Tropites. The position of the siphuncle is shown (as in Fig. 2) grrdually edging to the periphery of the whorl. The septal necks are seen at first to be backwardly-directed; then (at the 9th septum) to be both forwardly-and backwardly-directed; and, finally (at the r yth septum) to be only forwardly-directed. (Adapted, after Branco, from Zittel's Text-book of Palreontology) , stocks of ammonoid" which were derived from Bactrites in Palreo­ zoic times. It is enough to indicate the general plan of evolution that was carried out repeatedly in the different lineages. There was one group of Palreozoic ammonoids, however, that apparently cannot have been derived from Bactntes, and probably repre­ sents an independent divergence from the nautiloids. This group is the Gastrocampyli, and is characterised by having its siphuncle placed on the dorsal side of the conch-that is, in a coiled shell (as are all known Gastrocampyli), on the inner side of the whorl. In all other ammonoids, the siphuncle is ventral, that is on the outer side or periphery of a coiled shell. In all THE EVOLUTION OF A~ElIONITES. 57 the main lineages of ammonoids, the shell, so far as the evidence goes, passed from a bactriticone, through a mimocone, to an ammoniticone; the siphunc1e took on a ventral position, and either remained with backwardly-projecting septal necks (in the Microcampyli), or passed, often with an intermediate stage with both backwardly-projecting and forwardly-pro­ jecting septal necks (as in Tropites, fig. 7), to a stage with only forwardly-projecting septal necks; the ornament of the shell

8

9

10

II

12

rIGS 8-12. ST;TURES OF A"'IO';OIDS (after Zittel). Fie. 8. An asellate suture. Fig. 9. A latisellate suture. Fig. 10. An angustisellate suture. Fig, II. Suture of the Goniatite Glvphioceras, FIg. 12. Suture of Ceratites nodosus, became more complex; the ventral bay or sinus at the mouth of the shell was lost; above all, the earliest sutures, from being but slightly-curved iasellate, fig. 8), in the simplest forms, became latisellate (fig. 9), in most Palaeozoic ammonoid lineages, that is, having a single, broad saddle on the venter with a pair of deeper and broader lobes at the sides; or evenangustisellate (fig. 10), that is, having a prominent, sometimes sub-acute, ventral saddle with correspondingly deep lateral lobes. Forms belonging to those groups which are mainly latisellate are generally known as Goniaiites. Their suture during further evolution becomes more complex than that of a nautiloid, and often takes on finally 58 W. D. LANG ON the familiar zig-zag pattern of the typical Goniatite (fig. II). Another Palreozoic group, the Discocampyli, containing the well-known Ceratites with its peculiar adult suture-line (fig. 12), emhraces more primitive forms with their earliest sutures latisel­ late; and more specialised forms in which the latisellate passes into the angustisellate condition. So with the Phyllocarnpyli, a group containing the more primitive Ammonites. In some of these, the first suture is latisellate, and in others it is angustisellate; but almost im­ mediately the sutures of the individual become more complex, and exceed any forms which preceded them (except certain Discocampyli, for example, Pinacocerasi in the extreme specialisation of their suture-line. And here we are confronted with a matter of definition. In many Goniatite lineages, further developments occurred in which the sutures became far more complex than those of the typical Goniatite, and in some cases as complex, or even more so, than those of typical Ammonites. Are we to limit Ammonites to members of the Phyllocampyli and its derivatives, or to define an Ammonite as anv ammonoid with a foliated suture-line? 1£ the former, many simple-sutured Palaeozoic forms must be included; and, if the latter, those complex-sutured Palreozoic genera to which many Goniatites advanced may be called Ammonites, and an Ammonite may be considered as a Goniatite which has attained to a foliated suture. But the typical Ammonites are Jurassic and Cretaceous forms. And almost all, if not all, Mesozoic forms are Phyllo­ campyli or derivatives of this group, namely Leptocampyli and Pachycampyli. (For convenience, the Trias is included in the Palseozoic throughout this paper.) These invariably have complex sutures except in the earliest stages whose simple sutures, becoming progressively more complex, indicate the lineages along which the several forms have travelled. Gastrocampyli Mesocampyli MicrocampyliLEurycampyli Bactrites "'- GI O S S O C am pYI i. Glypltioceras ~ Tropites . /Discocampyli Primitive / Ceratites Phyllocampyli'x, /Leptocampyli "'-Phyllocampyli "-Pachycampyl Before leaving the ancestral ammonoids, it is of interest to note that the third division of Cephalopods, namely the belern­ noids, may be derived from ammonoid orthocones (bactriticones) THE EVOLUTION OF Ai\iMONITES. 59 by the reflexion of the mantle over the shell. and the secretion of a "guard" at the shell's apex. The chambered shell of belemnoids had a calcareous protoconch and a marginal siphuncle, and was in all probability descended from a Baetrites-like ancestor. Nearly all belemnoids in which the chambered shell persists are bactriticones; but in their later developments there is a ten­ dency [or the shell gradually to dwindle and finally to be lost. Thus the Recent Cuttles, Squids and Octopuses, being belemnoid, are more nearly allied to the Ammonites than is Nautilus. This conclusion runs counter to the orthodox classification 01 zoolo­ gists who unite Nautilus with the Ammonites as Tetrabran­ chiata, and class the Sepia-Lolieo-Ociopus alliance as Dibran­ chiata.

III. EVOLUTION ,\MONG AMMOXITES. It is evident that, whether we regard the complex-sutured Goniatite-derivatives as Ammonites, or confine that term to members of the Phyllocampyli and its developments, the adult Ammonite is already a highly-specialised form, and in many cases represents the acme of its lineage. Further develop­ ment in such cases is in the nature of catagenesis or declining evolution. Even of those primitive Ammonites which have a furtber history of specialisation before them, it is the characters of the early stages that have already sorted them into the main lineages to which they belong. And the multitude of Ammonite forms and even of Ammonite lineages is so great that on an occasion like this it would be hopeless to attempt even a general view, As in the evolution of forms leading up to the Ammonites, so in that of the Ammonites themselves, the same fundamental principles are seen, namely, Recapitulation. Orthogenesis and Independence of Characters. And it would probably be of more interest to examine particular examples which illustrate modifications of these principle'; than to attempt to deal with the evolution of Ammonites from the point of view of their sys­ tematics. We have already examined cases of recapitulation of orna­ ment, where a form having tubercles as an adult passes through, first a smooth, then a costate stage when young. It is noticeable how short is the smooth stage compared with the costate; and it may be generally stated that the ancestral stages are of shorter duration in direct proportion to their remoteness. This " speeding up " of remoter stages is called Taclrygenesis." and is well seen in other characters besides ornament. For instance the first suture of Tragophvlloceras loscombi (a form whose whole sutural history has been worked out by Spatht) is angustisellate

-A. Hyatt) 1893, Pn!c. Roston SCI(. Nat. nt«. vol, xxvi., p. 77. -i-L. F. Spath, '9'4, QlItl,t. Jourtt. Geol, Soc.• vol.lxx.. PP•.330-30Z. 60 W. D. LANG ON (fig. I3), the next few comparatively simple, or goniatitic (fig. I4), hut bv the time the shell has reached a diameter of only '2 mm. the sutureis already complex and ammonitic (fig. IS) ; and thence­ forth there is only differentiation of detail (fig. I6). Or, to take coiling--Aegoceras Iatacosia is evolute, and later developments of this species, known collectively as A. henlevi, become more and more involute. But the earliest stages of many of these forms are evolute, and this evolute stage is more and more condensed in the individual forms as development proceeds. A further modification of tachygenesis is illustrated by Ammonites of the same family as A. latacosia, This is a com­ plete skipping of a stage; not the fading off of a remote stage at the extreme apex of the shell, which is often the obvious

13

15

16

FIGS. 13-16. SL'TURES OF TRAGOPHYLl.OCERAS LOSCOMBI (J. Sowerby}. After Spath. Fig. 13. The first (angustisellate) suture. Fif!,. 14. The seventh (goniatitic) suture. Fig. IS. A later (ammonitic) suture. Fig. 16. A still later suture (at a diameter of 20 mm.).

result of normal tachygenesis, but the squeezing out of a stage between two others. It has lately been admirably demonstrated by Trueman* with regard to ornament in the group of Ammon­ ites indicated, which dunng phylogeny completely omit a previously-present costate stage. The skipping of stages is a modification of Tachygenesis and may be known as Lipogenesis, t

• Dr. Trueman's paper has not yet been published. It was read before the Geological Society on 24th April 1918. See ...J.h"had for that date, which also contains SOIne pertinent remarks by Mr. S. S. Buckman. 't Instead of the clumsy term Snltativ« Paliul!,(If('sisrS. S. Buckman, 1910, l·Olltshi'rt! Type Annnonitcs, part ii., PP. xiii, - xsv. Other examples arc there given. THE EVOLUTION OF AMMONITES. 61 It was explained above that characters such as those con­ sidered, whorl-shape, coiling, suture, ornament and the rest, were independent in their evolution, that is, they ran through their developments without regard to the stage reached by another character. Thus it is possible to have a form with comparatively simple sutures and a highly involute shell, or, conversely, one with complex sutures and an evolute shell. But imposed upon this independence of characters there is a certain correlation of characters in some cases. For instance, it has lately been insisted on by Spath* that the suture and whorl-shape are to some extent correlated; that in compressed shells, such as those of oxycones, the lateral saddles and lobes are compara­ tively more differentiated than the external saddles and lobes, and "that the opposite obtains in depressed shells; while all the saddles and lobes of the sutures in shells with a whorl cylin­ drical in cross-section tend to be equalised. Spath also suggests correlation between suture and ornament, as well as between suture and the attachment of the soft parts to the shell, and consequently the mode of of the Ammonite. Correlation of Characters, then, is a secondary principle imposed upon the fundamental Independence of Characters and modifying this primary principle in certain cases. A large part of the evolution of Jurassic and Cretaceous Ammonites is concerned with their catagenesis or decline. When these Ammonites are viewed as a whole, this is seen in the coiling of the whorl. The Pala-ozoic bactriticones and rnimocones have given place to ammoniticones ; and now, especially in Cretaceous times (fig 1J), a retrogression begins, and loosely-coiled criocones iCrioceras, fig. 1JC) appear, and in some cases straight forms-baculiticones (fig. IJa)-are attained, as in the genus Baculites. These baculiticones may be dis­ tinguished at once from bactriticones by the suture, which, though also much simplified by catagenesis, is still sufficiently complex. The torticone (fig. 1Jb), or asymmetrical spiral, is also a characteristically Cretaceous, catagenetic shell-form.] Similarly, a simplification of the suture-line is seen in viewing the Ammonites as a whole ; also a decline in ornament, finally reverting to a smooth shell. These characters, especially the last.are seen also in particular lineages, and reflected in the ontogeny ot individual shells. The evolution of every character was described above as if it Were a ",'ave, having a gradual upward slope, or ; an acme, or wave-crest; and a steep downward slope, or cata­ genesis. But it would be better to regard such a system as a period; since cases have been described which exhibit a Renewed

" 1919. Geol, Mag., Dec. 6. vol. vi., pp. 28-29. t Torticones occur at least as early as the 'frias-c.(., Cocliloccrcs. one of the Discocampyli. 62 W. D. LANG ON Anaeencsis. Such is Sonniuia renouata, * which renews a costate and, - finally, a tuberculate stage. In this case the sequence­ smooth-costate-tuberculate-costate--is seen to be but a period in a succession of similar sequences, and evolution is thus seen to be Periodic. This is in keeping with similar phenomena

c

Fig. 17. Catagenetic Cretaceous Ammonoids. a, Baculitcs, Danian; b, Turri­ tires, Albian; c, Crioceras, Neocomiau. \From British Museum Guide to Fossil Invertebrate Anirnals.) among Corals.r Polvzoa and Asteroideaj : and, at least in the first group, also suggests that recurring periods are not mere repetitions, but themselves show with the preceding

• See S. S. Buckman, A Monograpli of Ammonites of the Inferior Oolite Series, 1894, p. 433. and '905, p. cc. t W. D. Lang, '909. Proc. Zoot, Soc.. vol .. pp. 288-9· t W. K. Spencer, 1913, Plsil, Trails. Rcy•.\joe. series B, vol, cciv., PP.155-7. THE EVOLUTION OF AMMONITES. period a general plan of advance and decline; so that one system with its component phases of anagenesis, acme and catagenesis, is itself but a component of a higher system. This. again, is in keeping with a still more far-reaching principle in evolution, Which, for lack of better terminology, may be called the princple of transcendence, Whereby a new unit oj higher order is the result of a synthesis of units of a lower order*--a conclusion that joins hands with Hegelian philosophy. From the Ammonite to Hegel is an evolution indeed. There is yet time for a hasty catagenesis. Catagenesis inevitably suggests the of Ammonites. This is a subject which I would not discuss now; but rather leave it under these two considerations. First, renewed anagenesis shows that. cata­ genesis need not necessarily indicate final loss of vitality, but rather the periodic nature of vitality's expression. Therefore it is possible that under the term catagenesis two ideas are embraced; one is true catagenesis, a periodic manifestation {)f vitality, and the other a gerontic condition, or a manifestation of senility. Secondly. the fact that, in earliest Tertiary times, total extinction overtook all Ammonite lineages without regard to the degree of development each had attained shows that a general phyletic catagenesis or decline of the race cannot have been the only cause, or even the main cause, of their extinction. Many suggestions have been put forward to account for the complication of the suture-line. It is a fact that. generally speaking, the shells of Ammonites were extremely thin. And a very usual explanation of the suture-line is that by its com­ plexity it strengthened tne shell.t Doubtless it is true that a complex suture does strengthen the shell; but, while this would account for the elaboration, it would not account for extreme elaboration; besides, it does not suggest why the shell is so thin. It is probable that both phenomena are cor­ related with the bionomy of the Ammonite; and on this point nothing is known and but little surmised. Spath] has suggested that the complex suture helped to prevent the mass of the soft parts from falling out of the shell; but, again. a misfortune so awkward and so neatly averted would hardly result in the extreme complication of the suture. It has been postulatedf 1hat the variations of a useless structure are likely to be of systematic importance. since they would he more free from environmental influence. and thus would not be similarly modi­ fwd in unallied forms. Conversely, then, the variations of the suture-line, which have proved of the highest systematic

• H M. Bernard. 191:'. SOllie NCJ!.lc'dcd Factors ill Evolution, part II.; see also W D. Lang, 19]2. Gcot. Ma~. 5th dec, vol. ix , pp. 550-8. T See in this connection. L. F. Spath. 1919. Geol. lIfa#.!. 6th dec., vol. vi., PP. 1I9-20. :t: Geol. Mal!. IQI9. dec. 6 vol. vi., p. 30. ~ f.g., by E. Meyrlck, 18951 A Handbook of British. Lcpidoplcra, p. ) I. W. D. LANG ON importance, and, above all others, show what are believed to be th e true relationships 01 Ammonites, are likely to have proved particularly useless to their owners. A utilitarian explanation, then, is unlikely; and We are driven back upon one that implies a merely negative use, or even an inevitable burden. It was remarked that the Ammonite shell was thin; and it is likely that it evolved from a thicker shell. Now organisms which have taken to lime-secreti on appear to ex perience grea t difficulty in ceasi ng to deposit calcareous matter in such large quantit ies, and even in preventing its secret ion in progressively grea te r quantities.* If the Ammonite-shell is getting thinner (for reasons at present unknown), there will be a balance of calcareous matter which the Ammonite cannot help secreti ng and must deposit somewhere; and it is possible that by complicating the sut ure­ line, it achieved this without hampering it self elsewhere. It was an ingenious way out of a difficulty and could be carried on until the complexity became ext reme. It is worthy of note in this connection t hat the belemnoids could get rid of mu ch calc areous matter by further solidi fying the guard .t But at an ea rly date this group (like several lineages of Gastropod Molluscs) learned how to avoid secreting a shell and thus averted the calcium car bonate danger. Their reward is apparent at the present day. Among invertebrates th ey now dom inate the seas, and find the other branches of their order-the nauti­ loids represented only by an unspecialised survival, and t he ammonoids extinguished beneath their over-specialisation. Let us briefl y recapitulate. Nautilus, the only recent Ceph a­ lopod with an external shell, can be traced back to the Palseozoic, when it had numerous and variform relations. When the nautiloids have been followed backwards through loosely-coiled gyrocones, curved cyrtocones to st raight orthocones, Orthoceras is still found to be with out the calcareous protoconch invariably present in ammonoids. On the other hand, Orthoceras , or perhaps an OrlhoceYll.'i-like form, in whic h a chi ti nous protoconch had become calcified, gave rise to the straight ammonoid shell, or bactriticone, with its ventral siphuncle ; and this progressed along many lineages, through loosely-coiled rnimocones to ammoniticones. Ammonit icone lin eages with comparatively simple sutures are Gonia tites; and by further complication of the suture-line become Ammonites. The leadin g principles exhibited in this evoluti on are (I) Recapitulati on, (2) Ortho­ genesis and (3) Independence of Cha racters. Modifications of these have been considered in connection with the evolution ""'Se:e W . D. Lang, 1916, Gent. Mag ., dec . 6, vol. ii i., PP· 74-75' tSuch an expl anation of the gu ard appears m ore co nv inci ng than that wh ich would make it a protection for th e chambered shell when the belemnite shot backwards. with great velocity and was une xpectedly brought up agai nst a hard surface (see Hritish Muscllm Guide to Fossi t l nocrtebrutc Animals, 1907. p , 151 ), regardless of what the intervening­ hind er parts wo uld suffer ; or that wh ich co nsiders the guard to be a ba lanci ng we ight to keep horizontal a body which nat urally ten d. to stand on its head ; Re cent squids have no such balancing we ight. THE EVOLUTION OF AMMONITES. of Ammonites as such. Tachygenesis or accelerated develop­ ment, Lipogenesis or the skipping of stages and Renewed Ana­ genesis are special modifications of Recapitulation; and Correla­ tion of Characters is a secondary principle imposed upon and modifying Independence of Characters. Catagenesis is seen possibly to have been confused with senility, and not necessarily to be concerned in the causes of extinction. Renewed anagenesis is an expression of the principle of Periodicity in evolution, which in its tum marches with some ideas hitherto considered as belonging to pure philosophy. Finally, it is possible that the complexity of the suture, correlated as it is with great tenuity of the shell, may have some connection with difficulties ex­ perienced by the organism on acquiring the habit of calcium­ carbonate secretion.

PROC. GEOL. Assoc., VOL. XXX., PART 2, 1919.