OP THE ELASMOBKANCHS. 



69 



6 



s- 



Furthermore, with the exception of the family Heterodontidee (Cestraciontidse), as 

 defined by Mr. Woodward, no family of Elasmobranchs crosses the line dividing the 



Palaeozoic from the post-palseozoic world. 



It may be profitable to discuss the fate of the various Paleozoic families of Elas- 

 mobranchs, so far as we are able to reach conclusions regarding them from the materials 

 at our command. The Pleuracanthid* and the Cladodontidffi arc so di 111 'rent from all 

 other known sharks that they have been made to constitute a distinct order, the Ichthyo- 

 tomi. This order represents a distinct offshoot, which extends back as far at least as 

 the Devonian. The Pleuracani bidee bad their maximum of development in species in the 

 Coal-measures; the Cladodonthhc during the Subcarboniferous. In the States lyin 

 along the Mississippi river we find the species of the genus Chdodus pretty evenly dis 

 tribnted throughout the various divisions of the Subcarboniferous rocks, about thirty- 

 two species occurring therein to seven in the whole thickness of the Coal-measures of 

 the sai -egion. The upper division of the Subcarboniferous, the Chester, alone con- 

 tained five species. This shows that the genus was a declining one during the period of 

 the Coal-measures. The other genera of the family which existed during the Subcar- 

 boniferous period felled to reach the period of the Coal-measures. The structure of the 

 Pleuracanthidee is quite well understood ; that of the Cladodontidee much less satisfacto- 

 rily ; but from what we do know of the structure and history of the two families, it is very 

 improbable that they were the ancestors of any known sharks of the post-paheozoic 

 world. 



The Cladoselachicte is a, small family of a single genus and a, very few species. They 

 are, with respect to their paired fins, very generalized ; with respect to their tads at 

 least, they arc highly specialized. They are not known to pass beyond the early Sub- 

 carboniferous ; ::: and no related forms are recognized thereafter. 



We have no evidences that the Acanthodii possessed any representatives after the 

 Permian period. The order had its maximum of development in the Devonian. The 

 typical genus, AcanthoesMS, continued on into the Permian, in which Fritsch has 

 recently discovered one or two additional genera, containing two species. The members 

 of the order differ so much from other Elasmobranchs that their relationships with the 

 Sharks have been doubted. They almost certainly left no post-paheozoic descendants. 



Of the numerous genera of the Petal odontkhe the only genus that continued on into 

 the Permian is Janassa. Here it is associated with Cope's Thoraeodus. Hasse has sug- 

 gested that Janassa has relationships with our modern rays ; but Jaekel, who has studied 

 this genus, concludes that it had nothing to do with the rays. The Petalodontidse were, 

 after the time of the Subcarboniferous, a declining group. During this period there 

 *Bean, B„ 1894, Trans. N. Y. Atad. Sel, xiii, p. 115. 



