HARDWICKE'S SCIENCE-GOSSIP. 



269 



resembling the latter, but readily recognised by the 

 bluntness of the cones ; H. rancostatus, having teeth 

 much elongated, — cones not often well defined, but 

 pointed, and the central cone usually not very 

 prominent ; and //. inedius, of which the tooth 

 (tig. 168) consists of a single cone, with rather indis- 

 tinct indications of secondary or lateral cones. The 

 Oolites have afforded several species, and also the 

 Purbeck and Wealden ; H. grossi conns (fig. 163) and 

 //. polyprioii range from the Stonesfield Slate to the 

 Wealden ; spines described as H. dorsalis characterise 

 the Purbeck and Wealden beds ; and numerous very 

 perfect skulls of H. basanus have been found in the 

 latter strata on the shore of Pevensey Bay. Hybodits 

 spines are occasionally met with in the Cambridge 

 Greensand ; the remains of //. Dubn'siensis are some- 

 what rare fossils of the Chalk ;* and from this forma- 

 tion, also, Agassiz has described fragments of a 

 spine under the name of //. stilcafns. 



Orodontid.e. 



The characteristics of the members of this extinct 

 family are very imperfectly known ; nothing beyond 

 their dentition and dermal tubercles has hitherto been 

 <liscovered, and there is no definite evidence at 

 present as to the external form, or as to whether they 

 possessed dorsal fin-spines or not. It may be 

 questioned, in fact, whether the basis of the " family " 

 itself is secure, and whether De Koninck is not 

 nearer the truth in considering the group as a sub- 

 division of the Cestraciontida;. 



Orodus is regarded as the typical genus, and 

 appears to be exclusively confined to the Lower 

 Carboniferous formations, both in the Old and New 

 World, although Agassizodus, of the American Coal 

 Measures, is closely allied and probably its represen- 

 tative in that higher horizon. The best known 

 British species is O. ramosus (fig. 169), from the 

 Carboniferous Limestone. This tooth is elongated in 

 shape, sometimes as much as four inches in length, 

 and has the crown slightly raised into a blunt cone 

 with sides gradually sloping to the extremities ; it is 

 also characterised by a more or less prominent 

 longitudinal ridge, and its surface is embellished with 

 a number of ramifying lateral ridges, which are so 

 arranged as to constitute a most elaborate ornamenta- 

 tion. The larger specimens from Oreton (Salop) and 

 Bristol are, indeed, among the most beautiful of 

 fossils, but, in a palaeontologist's estimation, the 

 remains are all of a very unsatisfactory nature. 

 Never more than three or four teeth in consecutive 

 series have yet been discovered, and the same remark 

 applies to the other genera from British strata 

 referred to the same family. It is interesting to note, 

 however, that a nearly perfect jaw of Agassizodus has 



* See "The Geologist," Vol. VI, p. 2ji. 



been described* from the American Coal IVIeasures 

 and upon it alone we must depend at present for our 

 knowledge of the arrangement of the teeth in tlie 

 mouth of Orodontidse. This remarkable specimen 

 exhiliits nearly 500 teeth in their natural order, 

 and indicates that their disposition was very similar 

 to that observed in the living Ceslracion, but that tlie 

 rami of the jaw formed a much more obtuse angle at 

 the symphysis than is the case in the latter genus. 

 The majority of the teeth themselves are distinctly 

 Ilybodont in shape, like our Orodiis, while those 

 occupying the symphysial area are very similar to 

 (or possibly identical with) the little conical bodies 

 known under the name of Pctrodus (fig. 172) when 

 found detached. 



Some of the species of Agassiz' genus, Hclodns, are 

 also included in this family, and the well-known 

 names of Hdodiis mammillaris, H. didymiis, IF. 

 hcvissiinics, &c., must henceforth be replaced by 

 Lophodiis mammillaris, L. didymns, &c., if the most 

 recent ideas on the subject are to be accepted as 

 correct. It has been shownf that Hclodus originally 

 comprised two very different types of teeth, — one 

 distinctly conical, with a concavity of the base of the 

 crown corresponding to the convexity of its surface, 

 the other with a nearly even coronal base-line, 

 whatever the contour of the tooth. Those of the 

 former type are transferred to the Orodont LopJiodits ; 

 and those of tlie latter are provisionally retained 

 under the old generic name, although it is not unlikely 

 that future researches will prove them to be symphy- 

 sial teeth of Cochliodus, Psephodus, and other shark 

 of the Cochliodont family. J 



COCHLIODONTID.E. 



Perhaps the most typical Selachians of the Carboni- 

 ferous period are those referred to Owen's family of 

 Cochliodontidre. § Their remains seem to be exclu- 

 sively confined to Carboniferous strata, and are most 

 abundant in the lowest divisions, although a few — 

 such as Flemvdiis— occur also in the Coal Measures, 

 and attain their greatest development in that 

 formation. The group comprises a large number of 

 forms, no less than twelve genera having Ijeen 

 described from British rocks alone, and hence it 

 will only be possible to notice here very few of 

 them selecting those that arc most j^rominent and 

 characteristic. 



Cochliodus gives its name to the family, and is at 

 present known solely by the dentition. There is no 

 evidence yet forthcoming by which any of the 



* " Geol. Surv. of Illinois (Palaeontology)," Vol. VI, pp. 

 311-318. 



t H. Romanowsky, " Bull. d. 1. Soc. Imp. des Nat. de 

 Moscou," 1864, p. 160 ; L. G. de Koninck, " Fauna du Calc. 

 Carb. d. I. Belgique," Pt. i, p. 42 ; J. W. Davis, "Trans. Roy'. 

 Dublin Soc," 18S3, p. 403 



X " Geol. Surv. of Illinois (Pateontoloey)," Vcl. II p. S8 



$ " Geol. Mag.," 1867, pp. 59-63. Pis. III. IV. '' " • 



