OF THE ELASMOBBANCHS. 



71 



post-palseozoic deposits,' and attained the height of their development during the Juras- 

 sic; although the two principal genera, Hybodm and Acrodus, had their maximum in the 



Triassic. It, is not beyond possibility that Hybodm occurs in the Permian, a few teeth 

 Prom that formation bavin- been assigned to the genus. It has indeed been carried back 

 by some paleontologists to the Coal-measures, and into the Subcarboniferous; but this 

 would give to Hybodm a term of existence equaled by no other vertebrate, genus, and a 

 term quite improbable. Furthermore, no other genus, either Flasmobranchian or of true 

 fishes, is known will, certainty to have passed from the Paleeozoic to Mesozoic times. If 

 Hybodm has done this it holds a unique position. A similar remark may be made 

 regarding the family Heterodontid*, as limited by Mr. Woodward. If the Carbonifer- 

 ous genera are included in it, the family has come down to our day from the beginning 

 of the Subcarboniferous period ; and Rohon describes Rhabdiodm, said to be a Hybo- 

 dontid, from the Upper Silurian. No other family of fishes can claim such a history. 

 We would have to go to the invertebrates to hud a family that has existed so long. The 

 assignment of the Palaeozoic genera, to the family is based wholly on similarities in the 

 teeth; m.d this character is confessedly unreliable in sharks living in far removed 

 periods of time. Within a few years we have had it announced that in Chlamydoselachs 

 a living genus of Cladodonts had been discovered; but the error was soon dispelled. 

 Authors differ much in their disposition of the genera, here concerned. Dr. Zittel rec- 

 ognizes a family I Lybodonti.h.e in which are put Hybodus, Cladodus and a number ol 

 Subcarboniferous genera; while Orodus, Campodm, Arm,!,,,, etc., are put in the Ces- 

 traciontidse. Jaekel would exclude from the latter family Orodus, Gampodus, etc. 

 Therefore, I do not believe that we shall lose anything by retaining at least the genera 

 of the Carboniferous age in a, family with a, distinct name, Orodontidse ; and this I have 

 done in the fable of genera. Some day a, fortunate discovery of some member of the 

 group will reveal its relationships to modern forms. And 1 believe that I do not err in 

 Baying that the discoveries hitherto made of the skeletons of Paheo/.oic sharks have 

 proved their possessors to have been quite different from modern forms. As proofs of 

 this assertion may be mentioned Cladodus, Cladoselaohe and Pleuracanthus and its allies. 



However, if' would be rash to affirm that the post-palseozoic Heterodontidee have not 

 been derived from some member of the OrodontS. The latter family is apparently the 

 least specialized of the ancient groups, and some one of its less differentiated and more 

 plastic genera may have furnished the progenitors, not oidy of the Heterodontidse, but 

 possibly of all the modern fun dies of sharks and rays. 



It might be possible, as already suggested, to account for the break in the continuity 

 of the genera and families of Elasmobranchs which occurred during the Permian and 

 earlier Triassic on the ground that the environment along the shores where deposits were 



A. 1'. S. VOL. XX. .1. 



