The trilobite Mucronaspis in the uppermost Ordovician of the Oslo Region, Norway

ALAN W.OWEN

Owen, A. W.: The trilobite Mucronaspis in the uppermost Ordovician of the Oslo Region, Norway. Norsk Geologisk Tidsskrift, Vol. 61, pp. 271-279. Oslo 1981. ISSN 0029-196X.

Mucronaspis mucronata (Brongniart) and allied taxa are common uppermost Ordovician trilobites throughout the world. A lectotype for M. mucronata mucronata is chosen and illustrated, and the Norwegian subspecies, M. mucronata kiaeri (Troedsson), redescribed. Limited evidence suggests that dimorphism in glabellar proportions and palpebrallobe length is developed in the larger specimens and may indicate that sexual maturity is reached only in the later part of the holaspis phase.

A. W. Owen, Department ofGeo/ogy, The University, Dundee DD14HN, Scot/and.

Mucronaspis and the very closely allied Dalma­ nites' mucronatus but also proposed a new spe­ nitina are characteristic elements of late Ordovi- · eies 'D.' kiaeri for specimens with an unusually cian shelly faunas in many parts of the world. The narrow glabella. All existing museum material palaeoecological and palaeogeographical signifi­ and new collections have been examined in the cance of this has been discussed by several present study, which also includes some consid­ recent workers including Lesperance (1974), eration of holaspid morphogenesis. The speci­ Cocks & Price (1975), Lesperance & Sheehan mens are housed in the Paleontologisk Museum, (1976) and Jaanusson (1979). These authors have Oslo (PMO) and Hunterian Museum, Glasgow. also discussed the relationship of Mucronaspis/

Dalmanitina and associated trilobites to the Hir­ Terminology and techniques. - Most of the nantia brachiopod association. A similar study in morphological terminology used here is that the Oslo-Asker district is being completed by advocated in Moore (1959), except that the P. J. Brenchley and his colleagues. An unnamed glabella is taken to include the occipital ring new genus closely related to Mucronaspis is now (following Whittington l%2, p. 3) and the terms known as part of a dominantly graptolitic biofa­ rachis ( = axis) and dorsal furrow ( = axial fur­ cies in the uppermost Ordovician of Ireland and row) are preferred (Jaanusson 1956, p. 36-37). In Scotland (Siveter et al. 1980). addition, the term 'field' is used for the cheek Many of the Ashgill occurrences of Mucronas­ areas between the borders (Evitt & Tripp 1977) pis/Dalmanitina have been ascribed to M. muc­ and the glabellar Iobes and furrows are num­ ronata (Brongniart, 1822), a species based ori­ bered LI, Sl etc. forwards from the occipital ginally on material from the 'Dalmanitina' Beds furrow. in Vastergotland, Sweden. This species was re­ Specimens were measured under a binocular vised in detail by Temple (1952), who reported microscope using a scale graduated in O.l mm that the original specimens were missing. He intervals or, in the case of large specimens, using refrained from choosing a neotype, and the vernier calipers. The single orientation method syntypes have since been recovered and were for measuring specimens advocated by Temple illustrated by Reyment (1976, Fig. 9a) in a cata­ (1975) was adopted. A complete list of speci­ logue of types in the Palaeontological Museum at mens and measurements is included with the Uppsala University. The se specimens are refi­ figured material in the Paleontologisk Museum, gured here (Fig. l) and the cephalon UM Vg747 Oslo. Specimens for photography were coated selected as lectotype. The pygidium (UM Vg750) with dilute opaque and then whitened with is given paralectotype status. ammonium chloride sublimate. A photographic Norwegian material of Mucronaspis has been system similar to that described by Whittington known for over a century but has only been (1956) was used. illustrated in a descriptive work by Troedsson Family Dalmanitidae Vogdes, 1890. (1918), who ascribed most specimens to 'Dalma- Subfamily Dalmanitininae Destombes, 1972. 272 A. W. Owen NORSK GEOLOGISK TIDSSKRIFf 3-4 (1981)

B

Fig. l. Mucronaspis mucronata mucronata (Brongniart), 'Dalmanitina' Beds, Vastergotland, Sweden. Originals of Wahlenberg (1818) A. lectotype cephalon (UM Vg747), x 1.5. B. Paralectotype pygidium (UM Vg7 50), x 1. 5. Photographs kindly supplied by Prof. R. A. Reyment.

Genus Mucronaspis Destombes, addition, Mucronaspis commonly has a distinct preglabellar area and poorly incised lateral 1963. cephalic border furrows. Ingham's approach is Types species.- By original designation, Dalma­ followed provisionally here, although a numeric­ nitina (Mucronaspis) termieri Destombes, 1963 al taxonomic study similar to that recently used from the Upper Ktaoua Formation (Ashgill) in by Temple & Tripp (1979) for the encrinurids the Anti-Atlas region, Morocco. may well throw further light on the matter. Discussion. - Mucronaspis was first described as a subgenus of Dalmanitina Reed, 1905 by Destombes (1963), who later elevated it to full generic status (1972, p. 5(}....51). Campbell (1973) Mucronaspis mucronata argued that Destombes' original diagnosis of D. (Brongniart) kiaeri (Troedsson, (Mucronaspis) was only fully satisfactory for 1918) distinguishing its type species from that of D. f,ig. 3 A-R. (Dalmanitina) and that there was only limited O 1865 Phacops mucronata Brongn. - Kjerulf: 20. O 1887 justification for maintaining the two taxa. Ing­ Dalmanites mucronata Ang. - Brøgger: 33. O 1897 Dalmanites ham (1977, p. 111) gave an extensive review of mucronarus Ang. - Kiær: 33, 74. O 1897 Dalmanites mucro­ the problems involved and concluded that Muc­ natus Brong. - Kiær: 76. O 1902 Dalmanites mucronatus Brong. - Kiær: 79. 1918 Dalmanites mucronatus Brongniart ronaspis should be considered as a valid genus O - Troedsson: 71, 97 (pars- Norwegian material only). O 1918 with individual species being distinguished from Dalmanites kiaeri- Troedsson: 71-72, 97-98, Pl. 2, Figs. 23a, those of Dalmanitina by a combination of all or 23b, O 1934 Dalmanites mucronata - Strømer: 330. O 1981 most of the following morphological features: a Mucronaspis kiaeri (Troedsson)- Owen: Table l. O 1981 M. sp. - Owen: Table less convex cephalon, long genal spines com­ l. monly out of alignment with the lateral border, Lectotype (here selected). - A complete indi­ the posterior branch of the facial suture cutting vidual (PM066296, counterpart numbered the lateral margin of the cephalon at about the 66297). The specimen is from Lindøya, Oslo but leve! of the posterior border furrow. large eyes, a there is some doubt as to its precise strati­ less swollen glabella, Sl and S2 subparallel (cf. graphical horizon. Troedsson (1918, p. 72) gave distinctly convergent abaxially) with S2 only it as '5b' (=Langøyene Fm.) as does one of a well incised proximally, S3 less divergent fron­ number of labels with the specimen, all written tally, the posterior margin of the hypostoma at !east a decade after the specimen was col­ denticulate, thoracic pleurae extended as long lected (1891). Most of the other labels, including points, pleural ribs on the pygidium more curved one indicating the type status of the specimen, and more rearwardly directed posteriorly. In give the horizon as '5a' ( = Husbergøya Fm.). NORSK GEOLOGISK TIDSSKRIFT 3-4 (1981) The trilobite Mucronaspis 273 ' A c 30 30

X .!:! 25 .!:! 25 holotype ..ca; a; " 8 ..c .!:! 20 X 15 o 20 "' ::i c;. holotype/x ..ei holotype ..c: � o l X X X c:"' 15 c;. lO 'l;c: 15 ..c: .!! .!! cio � cio o lO ): 5 o lO ., >< .. '• �6 o c: - .o., 6 E ., � X 15 ., ., � ii holotype !. 'E �4 'x ., o � 4 �L X ..c ..c ..c holotype 15a. ., 'x X ., a. ..c: :=; c:() ·;: o 2 a. 2 'l;c: � :!. ., .. o.. ."c: _, o +--..---r---,.-----.-�---..---. ." a. o o o 5 10 15 20 25 30 35 o 5 10 15 20 25 30 35 sagittal length glabella sagittal length glabella

Fig. 2. Scatter plots for various dimensions in M. mucronata kiaeri. All measurements are in mm. 'x' = 'narrow' glabella morph, ' .' = 'broad' glabella morph. All specimens from the uppermost Ordovician of the Oslo-Asker district.

Occurrence. - The subspecies is common preoccipital length against maximum width (Fig. around Oslo in the uppermost few metres of the 2A) indicates that individuals with a pro­ Husbergøya Fm. and the lowest few metres of portionally narrower glabella are developed the overlying Langøyene Fm. (for stratigraphy amongst the larger specimens. This group in­ h see Brenchley & Newall 1975). In Bærum and cludes t e holotype and its implications are Asker it is less common, but is found at similar discussed below. There is no evidence of this stratigraphical levels which pass westwards into dimorphism in the degree of glabellar expansion the Langåra fm. The subspecies is not known compared with its posterior width (Fig. 2B) or outside the Oslo-Asker district. the proportion of the glabellar length occupied by the frontal lobe (Fig. 2C). All these plots Diagnosis. - Cephalon with well developed indicate virtually isometric growth and there is anterior border, long genal spines and large eyes. no clustering into distinct instars. Glabella ex­ Sl and S2 only very slightly convergent, the pands forwards to the mid parts of L3 in front of latter dying out abaxially. Two morphs based on which it is slightly constricted before increasing glabellar length to width proportions are de­ to its maximum width at the frontal lobe. The veloped amongst the larger specimens. Thoracic width at Ll is equal to 56-72% (mean 60%, pleural tips extended as spines. Pygidium with a standard deviation 4, n = 12) of the maximum distinct border, 9-13rachial rings and 7-8 (rarely glabellar width. Occipital ring tapering very 9) pleural ribs. gently abaxially, bearing a weak median swelling in its posterior half. Occipital furrow shallow and Description.- Cephalon gently convex (tr.) with transversely directed mesially, deepening and a sagittal length which varies from 50-70%of the directly rearwards abaxially. Sl transversely width at the posterior borders. This variation directed, S2 arched very slightly forwards such does not seem to be related to overall size. that L l tapers a little abaxially, deeply in ei sed Analysis of the outline of the glabella suggests mesially, dying out abaxially. L3 very slightly that two morphs may be present. A plot of sinuous, diverging forwards at 140-150°. Frontal

18 - Norsk GeologiskTidsskrift �81 274 A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 3-t (1981) lobe oval in the 'narrow' morph to lenticular in branches situated along the outer margins of the the 'broad' morph, having a sagittal length equal dorsal furrows and are convergent mesially on respectively to 76-83% (n = 4) and 45-68% the dorsal surface. Almost all the available head­

(n = 13) of the maximum width. Of the 6 broad shields are cephala, sugge sting that over the size specimens in the same size range as the narrow range present, the facial sutures were largely ones, this last percentage ratio is 55-67%. In non-functional. Eyes large, steep sided with both morphs, the frontal lobe occupies 53-65% 37-?40 files of lenses and a maximum of 8-10 of the preoccipital sagittal glabellar length (mean lenses per file. Field of free cheek narrow (tr.).

60%, standard deviation 3, n = 16). The smallest Lateral border broad, defined adaxially by no Cim spe ens have a (,listinct preglabellar furrow more than a break in slope in many specimens. and a narrow (sag., exsag.) portion of the anter­ Borders unite mesially as a narrow (sag., ex­ ior border on the cranidium. In larger specimens, sag.), flat anterior border. Genal spines long and irrespective of morph, this furrow is only very slender, diverging a little proximally but curving weakly incised laterally and dies out adaxially, round to become parallel. On the external sur­ where the anterior edge of the glabeila is defined face, cheek fields bear a dense, shallow pitting by the facial suture. External surface of glabella over which there is a fine, dense granulation bears a dense granulation which becomes finer which is also present on the borders and spines. with increasing absolute size and a median notch Internat moulds only bear a subdued pitting of on the posterior part of the frontal lobe. This the fields. notch becomes deeper on internat moulds and on Hypostoma known from a single, incomplete, larger specimens. The frontal lobe on internat specimen. Sagittal length equal to approximately moulds also bears a broadly triangular pattern of 125% of the width directly behind the anterior small swellings similar to those described by wings. Median body extending to anterior mar­ Eldredge (1971) in other dalmanitids. These gin, moderately swollen, oval in. outline with a swellings are only very weakly developed on the maximum width equal to about 70% of the sag­ external surface; internat moulds are otherwise ittal length. Maculae narrow (exsag.) but dis­ smooth. Dorsal furrows weakly incised but dis­ tinet. Crescentic posterior lobe occupying 25% tinet. Posterior border furrows deep and trans­ of the sagittal length of the median body. Ante­ versely directed over most of their length, but rior margin arched gently forwards. Lateral curving gently rearwards and shallowing distally wings tapering strongly abaxially. Lateral bor­ where they meet the poorly incised lateral bor­ ders ridge-like, definedby deep furrows. Posten­ der furrows. Posterior borders expand abaxially or borderflat-lying, definedanteriorly by a broad, (exsag.). Palpebral lobes gently inclined from the shallow furrow. Posterior border on! y partially field of the fixed cheeks; the anterior ends preserved but there is an indication of a small abutting the dorsal furrows immediately behind spine base a short distance out from the sagittal the anterior ends of L3, the posterior ends line. Ventral surfaceof median body bears large, terminating a short distance away from the scattered granules. Wings and borders smooth. glabella. The distance between the ends of each Thorax of Il segments, seen only in the palpebral lobe increases only slightly with in­ holotype. Rachis tapers rearwards at 15°, strong­ creasing cranidial size (Fig. 2D). This relative ly convex (tr.), occupying approximately 30% of shortening is reflected in the distance between the width of each segment. Articulating half­ the palpebral lobe and the posterior border fur­ rings equal in length (sag.) to about 50% of each row (Fig. 2E), the lobes extending rearwards to rachial ring. Dorsal furrows weakly impressed opposite the mid-parts of LI in small specimens but there is a marked break in slope here. but only to opposite Sl or even the posterior part Pleurae arched gently upwards, transversely of L2 in larger individuals. Such chang es are well directed proximally, laterally curving rearwards documented in Mucronaspis (e.g. Temple 1952, through about 45° in the posterior segments, p. 12). Although there are only limited data, much less so anteriorly. Distal parts of pleurae there is a suggestion that the palpebral lobes are extended as spines, those on the first few seg­ proportionally longer in the 'narrow' glabella ments are very short but the remainder are much morph (Fig. 2D) than in the broad form. Post­ more elongate. Pleural furrows deep, transverse­ erior branches of facial suture arched strongly ly directed, dying out and curving rearwards a forwards but terminating opposite the proximal little at the fulcrum. Pleural bands approximate­ parts of the posterior border furrows. Anterior ly equal in width (exsag.) and bearing a fine, NORSK GEOLOGISK TIDSSKR!Ff 3-4 (1981) The trilobite Mucronaspis 275 dense granulation, best seen on the extemal The limited data on proportional palpebral lobe surface. Rachial rings much more sparsely length differences between the dimorphs (Fig. granular. 2D) also support this view. Pygidium shield-shaped in outline with a sagit­ The morphogenesis of a closely allied subspe­ tal length (excluding mucro) equal to 84-88% cies, M. mucronata olini (see discussion below (o= 4) of the maximum width. Convex (tr.) for taxonomic status), is well documented rachis has a maximum width equivalent to 30% (Troedsson 1918, Temple 1952 a, 1957), and it (o= 2) of that of the pygidium and tapers rear­ seems most likely that the entire size range seen wards at 20o over most of its length and at 60° in M. mucronata kiaeri Iies within the holaspis over its posterior15%. 9-11 rachial rings may be phase of development (perhaps extending back developed along with a short terminal piece, or into latest metaspis). Thus the dimorphism, de­ 12-13 rings may be present without a terminal veloped only in the !argest individuals, is res­ piece. Rachis extended as a stout spine of un­ tricted to the later part of the holaspis period. known length which is directed rearwards and Scatter plots based on glabellar measurements of very gently upwards. Dorsal furrows shallow material assigned to 'Dalmanitina' mucronata and narrow (tr.). Pleural lobes arched: flat-lying from the upper Ashgill of Kazakhstan (Apollo­ proximally but fair! y steeply declined distally. nov in Apollonov et al. 1980, Text-Figs. 20, 21) 7-8 pleural ribs present in most (13) specimens indicate similar proportions to the 'broad' but one is known with 9 ribs. Interpleural fur­ Norwegian form and no dimorphism. rows deep and narrow. The anterior ones are Polymorphism amongst trilobites is well directed trånsversely over most of their length, known (see Campbell 1977: 10-11 for general curving rearwards a little beyond the fulcrum. discussion) and there are several cases ascribed The more posterior ones become almost parallel. to sexual dimorphism (e.g. Alberti 1971, Clark­ o Pleural furrows broader than interpleural fur­ s n 1%9, Hahn & Hahn 1971, Hu 1971, Jell rows, directed abaxially from just behind the 1975, Selwood & Burton 1%9, Temple 1975a). anteromesial corner of each rib to near the Most such studies comprise little or n� informa­ posterolateral corner. A distinct but narrow tion on morphogenesis and even in those that do, pygidial border is present. The external surface the implication is that dirmorphism is present at of the pygidium bears a dense, fine granulation !east throughout the holaspis period. The case (except in the furrows) but internal moulds are described here, therefore, differs in suggesting smooth. that M. mucronata kiaeri did not reach sexual A pygidium from S. Skjærholmen, Oslo (Fig. maturity until well into the holaspis stage. If an 9G) shows an interesting malformation - the analogy with another arthropod group, the coalescence of furrows from a pair of ostracods, were to be drawn, the dimorph show­ adjacent ribs on one side of the pygidium. This ing the continuation into adulthood of the juve­ specimen and other incomplete material is !abel­ nile morphology - in this case the 'broad' led as being from the unit below the Husbergøya glabella form - would be the male and the narrow Fm. and was collected on an excursion in 1914. glabella would typify the female. It is not possi­ The lithology, however, is that of the uppermost ble to as sess the validity of such an analogy. Husbergøya Fm., and this is assumed to be the correct horizon. Discussion. - Troedsson (1918, p. 71) noted that he bad examined specimens from Norwaywhich Dimorphism. - As is noted above, four of the previously bad been ascribed to 'Dalmanites' available cephala of M. mucronata kiaeri have a mucronatus, and he argued that white most greater glabellar length to width ratio than others belonged to this species, those from '5b' (the · of comparable size. The ·'narrow· and 'broad' LangØyene Fm.) of Lindøya and Skjærholmen forms occur together and are not confined to an y warranted specific separation. He distinguished particular part of the short stratigraphical range his new species, 'D.' kiaeri, from 'D.' mucrona­ of the subspecies. The restriction of the 'narrow' ta in ha ving a more elongate outline, its narrower morphotype to the larger specimens and the glabella and frontal lobe, in the shape of the position of these on a single growthcurve strong­ dorsal furrows, in the size of the eyes, the course ly suggest that the forms represent biological of the facial suture and the breadth of the entities and do not simply reflect tectonic or anterior border. In his extensive revision of diagenetic elongation parallelto the sagittal line. 'Dalmanitina' mucronata, Temple (1952) tenta- 276 A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 3-4 (1981) NORSK GEOLOGISK TIDSSKRIFT 3-4 (1981) The trilobite Mucronaspis 277 tively included the earlier references of Norwe­ p. 119), but they appear to be mutually exclusive gian material in his synonymy but excluded at other horizons and also in the Swedish succes­ without comment Troedsson's species. Kielan, sions. There has been considerable discussion however (1960, p. 119), synomymized 'D.' kiaeri about the significance of this (Temple 1952, p. with 'D.' mucronata. 26-28, lngham 1977, p. 113). Temple argued The lectotype ofTroedsson's species is a 'nar­ that, under some conditions, variation in M. row' specimen and thus many of his comments mucronata mucronata approached the condition relate to its differences from the 'broad' dimorph seen in a narrowly defined M. olini. lngham and possibly to some differences in absolute concluded that M. olini was derived originally size. There is very little to distinguish the 'broad' from an end member morph within M. mucrona­ glabellar form (including the small specimens) of ta. From the published descriptions and illustra­ the Norwegian taxon from specimens of M. tions, it would seem that although minor, the mucronata mucronata. The only consistent fea­ differences between the two forms are consis­ tures distinguishing all the Norwegian material tent, in spite of the wide range of variation seen from D. mucronata mucronata are the develop­ in M. mucronata mucronata at some horizons. ment of a broad (sag., exsag.) anterior border and lngham's suggestion of two subspecies with fair­ a slightly wider range in the number of pygidial ly subtle environmental controls on their relative rings and ribs. These differences, together with distribution seems to be the most acceptable the development of dimorphism, are not consi­ taxonomic approach. dered sufficient for full specific separation, and M. mucronata brevispina (Temple, 1952) is a thus M. mucronata kiaeri is viewed as a geo­ poorly known form based on specimens from graphical subspecies. basal Silurian strata in northem England (see lngham (1977, p. 113) considered that M. olini also Temple 1969, p. 228-229). The only pub­ (Te ....ple, 1952) may prove to be no more than a lished illustration of the cranidium is a specimen subspecies of M. mucronata distinguished only (Temple 1952, Pl. 5, fig. 6) which is smaller than by its shorter (exsag.) palpebral lobes and fewer any known individual of M. mucronata kiaeri pygidial rachial rings (commonly 9) and pleural and thus detailed comparison is not possible. ribs (6-7). These differences also differentiate Nevertheless, the palpebral lobe appears to be Temple's form from the Norwegian subspecies. proportionally shorter (exsag.) and more for­ M. olini and M. mucronata mucronata are found wardly placed with its posteromesial part further together at some levels in northem England from the glabella than in the smallest Norwegian (lngham 1977, p. 113) and Poland (Kielan 1960, specimens. Pygidia appear to be very similar.

Fig. 3. Mucronaspis mucronata (Brongniart) kiaeri (Troeds­ K. Dorsal view of partially exfoliated pygidium (P M011904), son). A-D Leetotype. Original of Troedsson (1918), Husber­ Husbergøya Fm., Husbergøya, Oslo, x2.0. gøya or Langøyene Fm., Lindøya, Oslo. A. Dorsal view of L. Dorsal view of intemal mould of eephalon (P MOI01542), east of extemal mould of eephalon (P M066297), 'narrow' 'narrow' morph, 1-5 m above base of LangØyene Fm., Gres­ morph. x 1. 2. B. Dorsal view of east of external mould of sholmen, Oslo, x1.2. thorax (P M066297), xI.l. C. Dorsal view of intemal mould of M. Dorsalview of partially exfoliated eephalon (P MOI01541), pygidium (P M066296), xI. l. D. Oblique la�eral view of inter­ 'broad' morph, same horizon and locality as L, x1. 0. nal mould of eomplete individual (P M066296), X1.2. N. Ventral view of ineomplete pygidium showing muero E. Ventral view of intemal mould of hypostorna (P M080491), (P M0103976), lowest part of Langøyene Fm. , S. W. Hove­ HusbergØya Fm., N. W. Hovedøya, Oslo, x2.0. dØya, Oslo, x1.5. F. Dorsal view of intemal mould of eranidium (P M0100830), O. Dorsal view of partially exfoliated pygidium(P M0100885), 'broad' morph, 1. (}..1.5 above base of LangØyene Fm., S. W. Langåra Fm., Marsehmanns Brygge, Blakstad, Asker, Hovedøya, Oslo, x1.2. X 2.5. G. Dorsal view of east of abnormal pygidium (P M011678), P. Dorsal view of small eephalon (P M020535), 'broad' morph, probably uppermost part of HusbergØyaFm. , S. Skjærholmen, 2.6 m above base of Langøyene Fm. , Husbergøya, Oslo, x 3.8. Oslo, x0.9. R. Dorsal view of partially exfoliated eephalon (P M0101549), H. Dorsal view of intemal mould of eephalon (P M020540) 'broad' morph, 4.3-0.8 m below top of Husbergøya Fm. , 'broad' morph, Langøyene Fm., Husbergøya, Oslo, x2.8. RambergØya, Oslo, X2.5. l, I, Q. Frontal, dorsal and oblique lateral views of intemal mould of incomplete eephalon (P M011059), Langåra Fm., S. W. Langåra, Asker, all x 2. 0. 278 A. W. Owen NORSK GEOLOGISK TIDSSKRIFT 3-4 (1981)

As Ingham (1977, p. 112) noted, M. mucronata Eldredge, N. 1971: Patterns of cephalic musculature in the Phacopina (Trilobita) and their phylogenetic significance.J. mucronata closely resembles the type species, Paleont. 45, 52-67,Pls. 13, 14. M. termieri (Destombes, 1963). The Norwegian Evitt, W. R. & Tripp, R.P. 1977: Silicified middle Ordovician subspecies differs from M.termieri in possessing trilobites from the families Encrinuridae and Staurocephali­ a broader (sag., exsag.) anterior cephalic border, dae. Palaeontographica Abt. A 157, 109-174,Pls. 1-24. the glabella expanding forwards more, S2 much Hahn, G. & Hahn, R. 1971: Revision von Griffithides (Bol/an­ dia) (Tril.: Voter-Karbon). Palaeontographica Abt. A 137, shorter (tr.), longer more stender genal spines 109-154,Pls. 25-27. and commonly more pleural rii:ls (7-9 cf. 6-?7). Hu, C. H. 1971: Ontogeny and sexual dimorphism of Lower Palaeozoic Trilobita. Palaeontographica Americana 7, 31- Acknowledgements. - This study was largely carried out 155. during tyhe tenure of a NATO postdoctoral fellowship at the Ingham, J. K. 1977: A monograph of the upper Ordovician Paleontologisk Museum, Oslo and was completed with the trilobites from the Cautley and Dent districts of Westmor­ assistance of a fieldwork grant from the Carnegie Trust for the land and Yorkshire. Palaeontogr. · Soc. [Monogr.] (3), 89- Universities of Scotland. I thank Drs. D. L. Bruton and D. A. 121,Pls. 19-27. T. Harper for their helpful comments on a draft of this Jaanusson, V. 1956: On the trilobite genus Celmus Angelin, manuscript. Drs. J. F. Bockelie, P. J. Brenchley and G. 1854. Bull. geo/.lnstn. Univ. Upps. 36, 35-49,Pl. l. Newall kindly placed their specimens of Mucronaspis at my Jaanusson, V. 1979: Ordovician. pp. AI3&-Al66 in Robison, disposal (now PMO). R. A. & Teichert, C. (Eds.) Treatise on lnvertebrate Palaeontology Part A.lntroduction. 569 pp. Lawrence. 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