HARDWICKE'S SCIENCE-GOSSIP. 



271 



in the Northumberland Coal Measures.* Usually the 

 teeth (fig. 170) are only met with in an isolated state, 

 but the very complete fossil just referred to is said to 

 show that there were three of these on each side of 

 the jaw, that the body of the fish was covered with 

 shagreen, and that at least one dorsal spine was 

 present. The spine is described as being short, stout, 

 and marked with prominent longitudinal ridges. 



Other genera of Cochliodonts are Strcblodics, 

 JDcItopiychiiis, Pcedlodus, Tomodus, and Xystrodiis, 

 and the species of Hclodiis (restricted) are also 

 associated with these in the most recent monographs 

 on the subject. 



Lastly, considering the Cochliodontidas as a whole, 

 it is evident that they form a family quite distinct 

 from any inhabiting recent seas, and there can be 

 little doubt that they are sharks somewhat approach- 



appears to be exclusively confined to the Lower 

 Carboniferous. The teeth are more or less flat, with 

 the root deeper than the crown, and the surface 

 either smooth and punctated, or covered with fine 

 rugose markings. In form, they vary considerably, 

 being occasionally much elongated (tig. 177 /'), and 

 sometimes triangular (ih., c) ; but generally they are 

 irregular parallelograms (ib., a), with the long axis 

 transverse, the hinder border somewhat concave, the 

 anterior slightly convex, one lateral margin at right- 

 angles, and the other obliquely inclined to the 

 anterior border. Though the diagram is rather 

 hypothetical in some respects, we may assume that 

 they were arranged in the mouth in the manner 

 shown in fig. 177, as suggested by Professor De 

 Koninck* and Mr. Davis, f and many palaeontologists 

 are now of opinion that the teeth with a smooth 



Fig. 170. — Plcnro- 

 dns affinis. 



Fig. 174. — Delicdns 

 sublcevis (lower 

 tooth). 



Fig. 178. — Copodus 

 coriiutiis. 



' Fig. 171. — Lnpho- 

 dus }nam>nillaris. 



Fig. 175. — Psepho- 

 dus magniis. 



Fig. 179. — Pleuro- 

 govzplms aiiriai- 

 latns. 



Fig. 172. — Pctrodus 

 patelliformis. 



Fig. 176. — Dcltodus 

 sublcevis (upper 

 tooth). 



Fig. 173. — Cochliodits contortiis. a, transverse 

 section of tooth. 



Fig. 177. — Diagram showing the probable 

 arrangement of teeth of Psammodus. 

 One-sixth nat. size. (After J. W. Davis.) 



ing the living Cestracion. It is noteworthy, however, 

 that the dentition bears some slight resemblances to 

 that of the Dipnoan Ccratodics, as was first noted by 

 Agassiz, and as has recently been discussed by Mr. 

 Davis, t and palaeontologists will anxiously look 

 forward to the discovery of more satisfactory specimens 

 to ascertain how far these surmises are correct and 

 how far Cochliodns and its allies may be deemed 

 " missing links." 



PSAMMODONTID.'E. 



This group, or family, is only represented in 

 British rocks by a single genus, Psammodtis , which 



* Quoted by John Ward, " Essays North StatTs. Nat. Hist. 

 Field Club," 1875, p. 223. See also Hancock & Atthey, "Ann. 

 & Mag. Nat. Hist." Ser. 4, Vol. IX., 1872, pp. 249-;52, and 

 J. W. Davis, "Quart. Journ. Geol. Soc," 1879, pp. 181-1S3. 



f " Trans. Roy. Dub. Soc," Ser. 2, Vol. 1, pp. 417-420. 



surface belong to the same species as those with the 

 rugose markings, — retaining only the name P. riigosiis, 

 and regarding P. porosiis as a synonym. 



COPODONTID.l-. 



The Copodonts are a most peculiar and pro- 

 blematical group of flat, crushing teetli, and sufficient 

 is not yet known about them to decide whether or 

 not theyconstitute anaturalfamily. Their relationships 

 are even more obscure than those of the Orodontida; 

 and Psammodontidre, but they have been provisionally 

 placed in a distinct subdivision of the Plagiostomi by 

 the author of the elaborate monograph, J already often 



* " Fauna du Calc. Carb. d. 1. Belgique," p. 41. 



+ " Trans. Ruy. Dub. Soc," 1883, p. 462. 



% " On the Fossil Fishes of the Carb. Limst. Series of Gt. 

 Britain," by James W. Davis, F.G.S.. in " Trans. Roy. Dub. 

 Soc," 1S83. 



