42 PISCES. 



form of tooth (fig. 32 B) has few cusps, often worn at the apex, 

 and the root or base is depressed. The typical Upper Cretaceous 

 tooth (fig. 32 c) is more elongated and has the root or base 

 slightly deepened and compressed. The most specialized tooth 

 of all, from the Pliocene (fig. 32 D), is still more elongated, with 

 from nine to eleven cusps, and with a remarkably compressed 

 deep base. The existing genus Chlamydoselache, which is very 

 closely related to Notidanus, is known among fossils merely by 

 detached teeth from the Pliocene of Italy. 



The remarkable resemblance between the teeth of Chlamy- 

 doselache and those from the Palaeozoic rocks named Diplodus 

 and Cladodus, originally believed to imply some close relation- 

 ship between the existing and remotely extinct genera, is now 

 proved to be of no significance for classificatory purposes. It 

 thus still remains uncertain whether the resemblance between 

 the dentition of the Palaeozoic family of Cochliodontidae and 

 that of the modern Cestracion, implies any very intimate 

 alliance between this ancient family and the Cestraciontidae. 

 The genus Helodus is the only undoubted Cochliodont hitherto 

 discovered showing remains of the body; and it exhibits at 

 least one spinous dorsal fin, while the pectorals are neither 

 much enlarged nor " archipterygial." An anal fin is also 

 present. Nevertheless the dentition of the Cochliodontidae 

 on the middle of each ramus corresponds exactly with that 

 of Cestracion, except that the separate teeth are more or less 

 completely fused together. Cestracion, as is well known, 

 exhibits two principal series of grinding teeth on the middle 

 of each half of the jaw (fig. 33, i), in which there are five 

 or six successional teeth behind (or within) the foremost row. 

 In the Cochliodont Psephodus (fig. 33, 2) the equivalent of the 

 hinder of these two series forms a great simple plate ; and this 

 is shown by abnormal specimens to consist of three teeth in 

 succession. In Pleuroplax or Pleurodus (fig. 33, 5), Deltodus 

 (6), and Cochliodus (3), each of the two series is fused into a 

 continuous plate, which always distinctly exhibits traces of its 

 components in the first two genera. In Pcecilodus (fig. 33, 7) 

 and Deltoptychius ( 4 ), not only do the two plates occur, but 

 they are once more fused together at their adjacent border 



