LtNNEAN SOCIETY OF lOKDON. 37 



The paired fins seem to resemble those of a modern Selachian, 

 and tlie anterior dorsal iin is provided with a spine. The 

 posterior dorsal tin seems to iiave been withont a spine, 

 as in the Chimaeroids. In the higlily specialised Cochliodont 

 Deltoptychius* there are tuberenlated spiny plates on the bead, 

 snggestive of some on the bead of the earliest known typical 

 Cbiinaeroid Mi/nacuuthus, wbich occurs in the English Lower Lias. 

 I^ may be added that the resemblance ot" the Cochliodont 

 dentition to that of tlie Chimaeroids has already been observed 

 both by Owen t and Egerton + ; and some of the Cochliodouts 

 and Chimaeroids have been grouped together by Jaekel § under 

 the name of Tracbyacanthidae. The so-called teeth of Chimaeroids 

 from Devonian formations (e.g., Pti/chodus) probably belong to 

 fishes of a vei'y distinct grouD. 



ISelachii. 



Traquair's discovery of Tristycli'ms\\ in tlie Calciferous Sand- 

 stone of Eskdale, Dumfriesshire, shows that there were already 

 ordinary sharks, with dibasal or tribasal pectoral fins and a 

 normal tooth-succession, at the beginning of the Carboniferous 

 period. Until Lower Jurassic times, all seem to have retained 

 the persistent notochord ; and the few well-preserved neural 

 arches which have been seen are long and well separated, without 

 any intercalary cartilages. Since then most of the true Selachii, 

 as I terui them, have acquired vertebral centra, and a consolida- 

 tion of the neural arches with intercalated cartilages ; most of 

 them (including even the most primitive, like Chlamydoselaclie, 

 fig. 1 n) also show various irregular fusions and subdivisions of 

 the cartilage-supports of the fins. They have also evolved into 

 more numerous families, and several parts of their skeleton have 

 become specialised in different ways. 



The Hybodonts (fig. 1 c), which for the most part exhibit the 

 primitiv^e notochordal condition until the Lower Cretaceous 

 period,' are especially interesting because, while their dentition 

 and their general ap|)earauce resemble those of the existing 

 Cestraciontidae, their skull is very different and more closely 

 agrees with that of the NotidauidjB H . They are indeed a 

 generalised group from which several later families appear to 

 have arisen, and they are the dominant sharks of the J urassic and 

 early Cretaceous periods. In Upper Jurassic rocks, however, we 

 begin to find good evidence of several modern families of both 



* A. S. 'Woodward, Proc. Greol. Soc. vol. Ixxi. (1915), p. Ixviii, text-fig. 2. 



t Odoutograpliy (1840), p. 65. 



X Quart. Joiini. Geol. Soc. vol. xxviii. (1872), p. 236. 



§ Sitzungab. Ges. Naturf. Freunde, Jierlin, 1890, p. 130; also ibid. 1891, 

 p. 127. 



II Geol. Mag. [3] vol. v. (1888), p. 83. 



*fr A. S. Woodward, Foss. Fishes Weald. & Purb. Form. (Mon. Palaeont. Soc, 

 191(5;, p. 6, text-fig. 3. 



