] ^6 CHIM.^'.ROin FISHES AND TllKIK 1)KVP:L0PMKNT. 



margin. These conditions are shown in lateral (outer) aspect in figures 126-129. 

 It may be added that there have been found (Eastman) a few detached plates 

 of Ptyctodus (in the Hamilton limestone) resembling those of Myriacanthus as 

 figured by Woodward (Cat. Foss. Fishes, vol. 2, pi. 2, fig. 2, ir). Also that in 

 Khynchodus the shape of the meckelian cartilage is known (fig. 127). These char- 

 acters, it will be seen, yield strong evidence in favor of their chimseroid nature. 

 On the other hand, we must admit the possibility they may yet have belonged to 

 some early specialized ofilshoot of a selachian stem which may not have given rise 

 to true Chimseroids. Thus they may have greater affinity with the Sandalodonts, 

 in which very similar tritoral points occur, or to Deltodonts or Cochliodonts, forms 

 which on fairly strong evidence are regarded as selachian. As to Ptyctodontids it 

 must frankly be admitted that there is nothing accurately known as to the form of 

 body, character of fins, and the jwssession of spines. In the latter regard, however, 

 it is fairly probable, as Eastman and others have shown, that the spine Phlyctjena- 

 canthus is to be regarded as belonging to Ptyctodus. And it is not impossible 

 that Belemnacanthus and Heteracanthus were associated with members of this 

 group. Harpacanthus and Cyrtacanthus may also have belonged to a Chimccroid. 

 But spines of this character, we must admit, might be associated almost equally 

 well with cestraciont sharks. 



The main virtue in the study of Ptyctodontids is to the writer this — that they 

 present some evidence (i) that Chimteroids are of Devonian stock; (2) that at this 

 early period their dental jilates were still but four in number, representing the 

 dental structures of the jaw halves of sharks; and (3) that the tritors existed as 

 small points forming together a texture in the dental plates which is well known 

 among early sharks. The evidence, in short, leads us to conclude with fair proba- 

 bility that the vomerine plates of Chim?eroids were a later acquisition. 



In connection with these earliest " Chim?eroids " there should be mentioned 

 the obscure group of Petalodontids, which occur abundantly throughout the Carbo- 

 Permian and were in some regards Chimsera-like, though it is more probable that 

 they represented forms of sharks which were not closely related to the ancestral 

 Chimseroid, Init were rather examples of parallelism. It is none the less noteworthy 

 that in such a form as Janassa the dental arrangement, although still retaining 

 discrete elements, suggests the formation of tritoral plates. Thus, we find that the 

 dental elements are crowded into the axial line of the mouth and are here provided 

 with interlocking ridges, which might well serve as the point of departure for the 

 evolution of tritors. In this event, the tritoral points would represent not each one 

 an individual tooth, but only a very small portion of a tooth. It may further be 

 shown that Janassa was singularly chimjeroid in the possession of a stout jaw, 

 thick and solid at the symphysis, and of remarkably large labial elements. Finally, 

 referring to Jaekel's reconstruction, it may be pointed out that Janassa possessed 

 a distinct antero-ventral fin lappet which appears to the writer to correspond more 

 accurately to the antero-ventral clasping organ of a Chimseroid than to an enlarged 

 fin ray of Raja, with which Jaekel compares it. In short, there is at least the 

 suggestion that in such a form as Janassa was represented a shark which had 



