CHIM/EROID FISHES AND THEIR DEVELOPMENT. 



INTRODUCTION. 



Chimseroid fishes, a group representing some of the oldest and simplest of 

 backboned animals, are considered in the present memoir with especial regard to 

 their relationship and descent. To this end, attention has now been paid to the 

 plan of their embryonic development, and upon this side evidence has been obtained 

 which, whether of major or minor importance in the study of descent, has at least 

 the interest of newness. For to the embryologist Chimaeroids have until recently 

 remained practically unknown, and they are thus the only vertebrate group of their 

 anatomical importance if ranked as a subclass to have escaped investigation. 



On the other hand, from the standpoints of comparative anatomy and paleon- 

 tology these shark-like fishes have received considerable notice, and they have 

 figured in publications of the past half-century as the "most primitive vertebrates," 

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

 jaw-bearing vertebrate. And in such a role (which I now believe is only partially 

 deserved) they have been given especial importance in problems of descent. 



The evidence which has been brought forward to demonstrate the primitive 

 nature of Chimaeroids is based in part upon the findings of paleontology ; it is, 

 moreover, as one frankly admits, supported by anatomical facts which are broad 

 in range and which have in many instances been provided by masters in morph- 

 ology. The substance of this evidence is that Chimaeroids, although shark-like, 

 are nevertheless widely distinct from the shark, and that they represent a lower 

 plane in piscine evolution. As an aid to subsequent reference, the grounds for this 

 conclusion may now be summarized. 



PALEONTOLOGICAL. 



Chimseroids are believed by some to be older than sharks. Their fossils, as 

 Walcott maintains, occur among fragments of "fish" plates in the Ordovician 

 (Lower Silurian) sandstones. Sharks, on the other hand, do not occur that is, 

 unquestionably before the Upper Silurian. Probable it is that Chimaeroids lived 

 during the Lower Devonian and, judging from their dental plates, these forms, if 

 Chimaeroid, were highly differentiated, even at this early period. Moreover, according 

 to the studies of Jaekel, paleozoic Chimaeroids provide the evolutional stages from 

 certain archaic armored "fishes" to the shagreened sharks. 



