446 THE GRAPTOLITES OF THE LOWER LUDLOW SHALES.

Dr. Henry Woodward* has stated that with one exception these forms of Astacoderma are founded on the teeth of Phyllopod Crustacea, such as Ceratiocaris. Dr. Harley also states that the Conodonts of Pander possess a similar minute structure to that of Astacoderma, but I have failed to find in the former the tubuli Which are so prominent in the latter. I have not recognised a single genuine conodont in any portion of the Bone-bed which has come under my notice. Another group of organic fragments very numerous in the Bone-bed, are bright, lustrous, black semi-cylindrical pieces of shelly bodies, with portions of tubes of similar material, from 3 to 8 mm. in length. These bodies are very conspicuous on the surface of the Bone-bed, and they have given rise to a fancied resemblance to broken up wing-cases of beetles. In micro­ scopic section they only show concentric lines of growth. In general appearance these fragments are not unlike the shelly tubes of Serpulites longissimus, Murch., which is of not infrequent occurrence in the Ludlow formation. The evidence seems to me insufficient at present to determine whether they are annelidan tubes or not. Of the remains of Brachiopods in the Bone-bed, mention has already been made of small black fragments of Discina. I have also noted one or two specimens of Chonetes and casts of other forms. Small black coprolitic nodules are frequent. Microscopic sections show that they are mainly composed of the minute siliceous grains of the rock matrix with occasionally a plate of Thelodus. It may be doubted if they are due to fishes.

THE GRAPTOLITES OF THE LOWER LUDLOW SHALES. By ETHEL :VI. R. WOOD. :VI.Sc. The Lower Ludlow Shales, as typically developed in the Ludlow district, constitute in the main an argillaceous group of light mudstones, greenish-brown to grey in colour. They are both underlain and overlain by a limestone series, the Wenlock and Aymestry limestones respectively. Although the Lower Ludlow formation as a whole is well distinguished lithologically from the Aymestry limestone above, yet there is a gradual transition from the one to the other, and the small calcareous fiaggy beds which are found in the lower part of the series increase in number and thickness towards the summit, until the Lower Ludlow Shales pass into the typical impure limestone known as the Aymestry. An examination of the palseontological evidence (at least so far as the •" Siluria," 5th Ed. (1872), p, 5;2. 'fHE GRAPTOLITES OF THE LOWER LUDLOW SHALES. 447 graptolites are concerned) brings out the same fact, namely, that there is no great break between the Lower Ludlow Shales and the overlying limestone, and they might be regarded more con­ veniently therefore as one formation. The light coloured mudstones of this Ludlow district bear but little resemblance to the black shales of Scotland and Wales, which are so characteristic of graptolitic deposits at other horizons. Graptolites, however, are found in abundance, especially in the softer mudstones and shales, and owing to their manner of preser­ vation, are exceptionally easy of examination and determination. Although specimens are abundant, yet they are as a whole small in size, and at first sight there appears to be but little variety of form. In fact two general types only strike the eye, namely, those that are slender and curved, and those which are fairly stout and straight. The same uniformity is to be found on a closer examination of the individual cells or thecse, Most of the apertures are either provided with spines, or are quite desti­ tute of ornamentation, and present the simplest type known among graptolites. A detailed palreontological study, however, reveals the ex­ istence of a large number of species and varieties. The differences between these forms are not so striking perhaps as those which are to be found among the graptolites of other formations (such as the Birkhill Shales, for example), but they are none the less constant-nor is it a matter of surprise that such should be the case when we reflect that the graptolites as a whole were approaching extermination in the Lower Ludlow Beds. The characters of most value in the separation of the species and varieties are (I) the form of the proximal end, (z) the form of the polypary, and (3) the shape of the thecse, Two genera have been found in the Lower Ludlow Beds of Great Britain, namely, J1:folZograptlts and Retiolites, but of these the former only has been found in the Ludlow district. The genus Monograptus is represented by fifteen well-marked species and thirteen varieties, which fall into the following six groups (only the commonest forms to be found in the Ludlow district are here mentioned) :

Group L-Type M. dubius. M. dubz'us, M. tumescens, lifo tumescens var, minor. Group H.-Type M: co/onus. iifo colonus, M. COlOtIUS var, compactus, iv1. uariaus, 1'11: varians var. pumilus, M. Roemeri. Group IlL-Type il:l. chima:ra. M. chima:ra, M. chima:ra var. Salwqi, Jl.f. leint­ zoardinensis, etc. Group IV.-Type 111. uncinatus. M. uncinatus val. micropoma. 448 THE GRAPTOLITES OF THE LOWER LUDLOW SHALES.

Group V.-Type 1I£. scanicus, M. scanicus, etc. Group VI.-Type M Nilssoni. M. Nilssoni, M bollemicus.

Of the six groups of Monograptus enumerated above, the first two-those of .111. dubius and M. colonus-are by far the most important, and both are rich in species and varieties. The species belonging to these groups, together with those of the two succeeding groups--those of M chimera and iT£. uncinatus -all bear a close resemblance to one another in the form of the polypary, which is straight for the greater part of its length, but distinctly curved inward at the proximal end. They differ from each other, however, in the character of their proximal extremities. and the shape of the thecre. In the first group-that of M dubius-the thecse are of the same type throughout the whole polypary and bear simple unornamented apertures, which are at right angles to the general direction of the thecae, Those of Group II.-that of M. colonus-bear two types of thecre, those or the distal end resembling those of Group I., and those of the proximal end having their apertures ornamented with spines. The structure of their proximal extremities also presents charac­ teristic differences. The forms belonging to Group IlL--that of iJ£. cllima:ra­ chiefly differ from those of the preceding group by the presence of spines, more or less stout at all the thecal apertures. The separation of this group from that of M colonus must, however, be regarded as provisional, for the presence of thecal spines seems in many forms to be dependent on external conditions, and can hardly be considered of great classificatory value. Group IV.-that of M. uncinatus-is only represented in this Ludlow district by one form, namely, that of M uncinatus var. micropoma, and that is a rare fossil. The two final groups-those of l/£. Nilssoni andM scanicus -are quite distinct from the four preceding ones in the form of the polypary, which is very slender and curved. The species belonging to them, however, differ from each other in the shape of the thecee ; those of the type of M. scanicus having a characteristic hooked or claw-shaped aperture, and those of M. Nilssoni a simple and unornamented one. Other characters, such as those of the proximal end, also serve to distinguish them. A comparison between the graptolites of the Lower Ludlow beds and those of the Wenlock below (which may be seen at Burrington and elsewhere in the Ludlow district) reveals some interesting facts. Regarding the graptolite fauna as a whole, it will be seen that the supposed great palseontological break between the Wenlock and Lower Ludlow Beds all but disappears; the same families and genera, and most of the groups of THE GRAPTOLlTES OF THE LOWER LUDLOW SH AL ES. 449

4JIonograptus are represented in both. At the same time they differ considerably when we come to study them in detail. Only one or two species are common to both, and it is no difficult matter to recognise at a glance whether we are dealing with a Wenlock or Ludlow fauna . The main characteristics of the Wenlock graptolites, which serve to distinguish them from those of the Ludlow beds are as follows: (I) The form of the polypary of many of the Mono­ graptids is straight with a curve outward at the proximal end . (2) The thecal apertures are more frequently curved over into a hook or claw shape, are occasionally simple and unornamented, but never spinose. (3) Many of the narrow forms give off branches and constitute the genus Cyrtograptus, The commonest graptolites in the Wenlock beds are .M. p riodon, M. Flemingii, jl£. dubius, and various species of Cyrlograptus. As we have seen, the Lower Ludlow fauna of graptolites is not a n extensive one, nor one of any great variety, the species are never of any great size, and to the collector it bea rs no com­ parison with the rich and varied fauna of the Glenkiln, Hartfell, BirkhilI, or even Wenl ock Shales. Yet it has a special interest of its own to the paleeontologist, on account of the fact that here we have the last survivors of that great family of extinct animals, which flourished exceedingly all through the Ordovician and most of the Silurian period, and died out suddenly, leavin g hardly a trace behind. A brief and partial summary of the gradual lines of evolution alo ng which the graptolites app ear to have passed, may give an add ed interest to the study of this small group of survivals. The earl iest graptolites, those in the Cambrian and lowest beds of the Ordov ician, were much branched forms ; many at first with the branches united together along the ir whole length (Dicryollcl1la),and later with the branchesfree (Clonograptlls). Graduallythe branches becam e more and more reduced in numbers (D ichograp tus and Tetragraptus) until eventually the re were only two left (Didymo­ graptus). In all these forms the shape of the thecee remained practically the same. Development and change now seem to have been centred in the shape of the thecal aperture, and various modifications took place, apparently for the better pro­ tection of the polyp living in the theca (Leptograptus and Dicelio­ graptus, etc.). A further simplification of the polypary resulted in the appearance of a form with only one branch ( jJ f onograptus). This genus reached its maximum in the Birkhill or Llandovery period, and included a large variety of forms , each characterised by a considerable variation in the form 450 THE GRAPTOLITES OF THE LOWER LUDLOW SHALES. of the thecal aperture. All the types represented in the Wenlock and Ludlow Beds are merely survivals of similar ones which reached a greater development in these earlier deposits. From the Birkhill period to the Ludlow there is a marked decrease in the number of forms; there is no sign of any new lines of devel­ opment, and the graptolites were clearly approaching their extermination. There was a revival of branching in the Wenlock beds (Cyrtograptus), but this modification of the simple un­ branched polypary was of short duration. In the Ludlow Beds, practically only one genus (.iJ1onograptus) survived, and the species were always small and showed an increasing diminution in size towards the top of the series. The reasons for this fairly rapid extermination of the whole family of graptolites are of great interest, although they cannot be determined with anything approaching to certainty. A possible reason may be found in the change of surroundings. In the earlier formations which were so eminently favourable to their exist­ ence, fairly deep and quiet waters prevailed and sedimenta­ tion was slow, whereas now these conditions were changing, for the Upper Silurian deposits indicate shallower water. Again, the great development of spines suggests another explanation, for it is possible that this may have been the expression of a necessity for defence against external foes, and it may be remembered that the great class of Fishes was beginning to enter as fresh and powerful competitors in the struggle for existence. Five main graptolite zones have been detected in the Lower Ludlow Beds of Great Britain, but of these only the four upper ones have hitherto been discovered in the Ludlow district. The zones are: (I) Zone of M. leintwardinensis, (2) Zone of M. tumescens, (3) Zone of M. scanicus, (4) Zone of M. Nilssont~ (5) Zone of M. vulgaris. The uppermost zone of M. ieinteoardinensis, which is typically developed in this district in the neighbourhood of Leintwardine, includes the Aymestry Limestone and the underlying calcareous laminated flags and light brown mudstones which form the upper part of the Lower Ludlow Shales. M. leintwardinensis occurs in this zone practically without associates. The zone of M. tumescens is well seen both along the Elton and Ludlow road, and also in the Elton Lane section, and con­ sists of light flaggy mudstones intermediate in character between thehard flaggybeds above and the softer shales below. M. tumescens occurs like M. leintzlJardinensis with few or no associates. The zones of M. scanicus and M. lVilssoni may be considered together in this Ludlow district, for they are intimately associated. THE SILURIAN CEPHALOPOD,\. 45 1 The rocks are for the most part thinly bedded mudstones and shales, and are far richer in graptolites than those of any of the other zones. Almost all the typical Ludlow forms are to be found in these beds. They are well exposed in the Elton Lane section, and an abundance of beautifully preserved graptolites may be collected there. As has been mentioned above, the lowest zone of M. vulgaris, which is well developed in the Long Mountain and Builth districts, where the limestone facies is practically absent, has not hitherto been found in the Ludlow district. It is possible that the horizon of this zone is represented by non-graptolitic deposits, perhaps, indeed, by part of the Wenlock limestone. A further examination, however, of other sections where the lowest beds of the Lower Ludlows are better developed will probably reveal the existence of this characteristic graptolite. Hitherto, the graptolite zones have been worked out only in one or two typical sections. Much remains to be done in tracing them throughout the whole extent of country along the great escarp­ ment of Wenlock and Ludlow Beds, which stretches from Much Wenlock to Kington, and still further south.

THE SILURIAN CEPHALOPODA.

Br THE REV. J. F. BLAKE, !'>I.A., F.G.S.

The Silurian rocks of Britain have yielded about 100 different forms of Cephalopod remains as at present determined; but as farther openings are made in the course of construction of public works the number will doubtless be enlarged, seeing that more than one out of every four of these have been established or modified through the excavations of the South Wales railway at Ledbury. In the growing population of the country the ordinary opportunities of collecting in such rocks are seriously diminished, whence it results that most of the Cephalopods available have been collected long ago, and in such cases the horizon is often unknown or left doubtful. Many also of the specimens on which determinations have to be made are only fragments of parts with some essentials wanting, worn away, pressed out of shape, or concealed so that the identity of two species from different horizons, or even the same from different localities, can only be relied upon as far as it goes, which may not be far. But after the last twenty years during which time zoning has been invading various parts of the Geological Series, and even hemera: have been attempted, the identity of two fossils on different horizons does not seem theoretically possible, though