I2 4 



CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



and arrangement; in fact, one might even go so far as to maintain that from a large 

 series of dental plates of one species of Chimaera one might obtain variants which, 

 separately considered, would be placed with other species. Moreover, from the 

 function of these crushing plates, it is not unnatural that marked differences should 

 appear in specimens of different ages and from different localities (e.g., from those 

 individuals which have lived upon different food material). In short, we incline to 

 the belief that changes in the dental plates of Chimaeroids do not predicate as wide 

 divergences in lines of descent as one would naturally expect. From the standpoint 

 of adaptation, furthermore, admitting the extreme value of physiological adaptation 

 in dental plates within the limits of the present group, we obtain a suggestion why 



phylogenetic changes 

 are not recapitulated 

 favorably in their devel- 

 opment. In a form, for 

 example, like Callo- 

 rhynchus, in which the 

 basal (trabecular) por- 

 tion of the plates has 

 become greatly devel- 

 oped in the adult, we 

 naturally expect that 

 there will be less oppor- 

 tunity shall we say 

 time ? for the tritors 

 to recur in develop- 

 ment in a separate and 

 finished form. If they 

 do appear, they appear 

 regularly only in 

 form, soon to be remodeled or erased. Thus we find in 



Fig. 1 04. Callorhynchus callorhynchus. Dental plates and neighboring mouth parts of late 

 embryo (about 1 10 mm. long). After Schauinsland. 



Fig. 105. Callorhynchus. Detail of middle ridge of mandibular dental plate of specimen 

 slightly younger (about 95 mm. in length) than the preceding. The dental ridge is seen 

 as a transparent object. After Schauinsland. 



Fig. 106. Callorhynchus. Dental plates of " larva " measuring about 16 cm. After spec- 

 imen in museum of Columbia University. 



"family" or in "generic 

 Callorhynchus, according to the figures of Schauinsland, that these tritors do occur 

 in later embryonic stages (fig. 105), although this author does not refer distinctly 

 to the relation of dermal cusps to tritors in Chimaeroid plates. Following briefly 

 the problem of the dentition of Chimseroids, we may again refer to the presence of 

 numerous papillae in the mouth region of these forms. For, by analogies in other 

 fishes, these structures may well represent rudiments of discrete denticles. It is, 

 therefore, of particular interest that in the case of Callorhynchus, where the dental 

 plates are heaviest and largest, we find a corresponding increase in the size of the 

 papillae. For it may be suggested that papillae which have become calcified either 

 singly or in groups, have retained their dentitional (and ancient) trend in evolution, 

 while those which remain soft have survived because they have undergone a 

 change of function. The similarity in dental and non-dental structures is shown 

 strikingly in the roof of the mouth of Callorhynchus (fig. 104), after Schauinsland. 

 That shown in the roof of the mouth of Chimaera (plate ix, fig. 5O e ), although not 



