IV. CHIM/EROIDS IN THE PROBLEM OF VERTEBRATE DESCENT. 



On the basi.s of the foregoing discussion we may finally consider the critical 

 question whether Chimseroids are to be regarded "as the most primitive verte- 

 brates, or more precisely as the least modified descendants of the ancestral cranium- 

 and jaw-bearing vertebrate ? ' ' Are they, in other words, to be looked upon as 

 more primitive than sharks and as "representing a lower plane in piscine evolu- 

 tion"? These questions have been touched upon, more or less distinctly, throughout 

 the present paper and the conclusion has been already indicated. And I think we 

 may now state confidently that, from the evidence of embryology and paleontology, 

 Chimjeroids represent not the ancestral vertebrate, but rather a highly modified 

 group descended from selachian ancestors. At the present time the evidence may 

 be summarized upon which this induction is based. 



PALEONTOLOGICAL EVIDENCE THAT CHIM/EROIDS ARE DERIVED FROM SELACHIAN 



ANCESTORS. 



(a) Their later origin: 



The earliest genera of whose Chimseroid nature there can be no doubt do not 



appear before the lower Jurassic, and from this horizon have been described but 



two genera. Sharks, on the other hand, appear in ages remotely earlier, and they 



are then represented by several orders, many genera, and very many species. 



Thus, in the Paljeozoic alone, we may enumerate at least fifteen genera and forty 



species whose shark-like anatomical features are definitely known, and we may 



reject altogether the testimony of the numberless selachian "species" of spines and 



teeth. Into this limbo of indeterminata may provisionally be cast Ptyctodonts, 



together with Cochliodonts, Deltodonts, and similar forms. And we may in like 



manner regard the Permian Menaspis as doubtful. But even if we grant that all 



Ptyctodonts are Chimseroid, we have still the testimony that the sharks were 



in earlier periods overwhelmingly more numerous and more diversified. And we 



have equally to admit that, even at that early period, many sharks, from horizon 



to horizon, modify the character of their cuspid teeth in the direction of tritoral 



plates. In short, admitting the evidence of dentition, one may state conservatively, 



that even in their epoch Ptyctodontids stood to the sharks, both in number and 



in variety, only as one to one hundred. And from this testimony alone we can 



almost reject the thesis that Chimseroids were ancestral sharks. Unfavorable to 



the latter view, moreover, is the fact that the culmination of the Chimseroid line, 



i. e., in genera and species, did not occur before the Cretaceous, while that of 



sharks antedated the Permian. 



151 



