J20 CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



already fused teeth, which had primitively passed from behind and taken up a position on the 

 dental ridges. In the case of these teeth (a similar process occurs in the ontogeny of Cerato- 

 dus) spongy dentine, or bone-like masses, were differentiated in the course of phylogeuetic 

 development, and these became finally of greater value for purposes of nutrition than the separate 

 teeth : and they accordingly fused together, overgrew the teeth, and in the end completely 

 enveloped them. And since the teeth had no longer their primitive function, they came to lose 

 their limy structure and degenerated, remaining in the condition in which we see them to-day. 

 While their arrangement in three rows possibly indicates an alliance with the higher forms, 

 their mode of successional growth suggests the origin of the rows of teeth of selachians. 



The results of the foregoing observations of Schauinsland, it will be seen, are 

 disappointing to those who on a priori grounds anticipated that the dental plates 

 of Chimaeroids would in the ontogeny of recent species be found to be formed of 

 the coalesced bases of separate tooth elements, which, in their turn, would of 

 course be homologous with those of sharks. One may, nevertheless, I believe, 

 take a somewhat more hopeful view of this problem, in view of the evidence 

 above provided. In the first place, however, in order that there may be a better 

 understanding of the terms of the problem, it will be found expedient to review 

 briefly the characters of dentition known among the more prominent types of 

 recent Chimaeroids, for there is room for the belief that Callorhynchus, in spite of 

 its many archaic features, may prove to have modified the conditions of its dental 

 plates, or at least parts of them (the "tritors")- more completely than some of the 

 other forms. 



To this end we may compare the dental characters of Harriotta with those of 

 Rhinochimsera, as representing extreme types in Chimjeroid dentition. In fig. 94 a 

 are shown in Harriotta the dental plates and the roof of the mouth; in fig. 94 b the 

 dental plates, tongue region and floor of the mouth, and, in figs. 94c and 94 d, 

 corresponding regions are shown in Rhinochima?ra. Contrasting these forms, we 

 notice that in Harriotta the dental plates are studded with peg-like eminences, 

 some of which, both in the upper and in the lower "jaws," form together tumid 

 tracts or ridges. These peg-like eminences, "tritors," are found to pass deep 

 into the substance of the dental plate; thus, where the plate is flattened and 

 more or less transparent, as at the anterior margin, the peg-like structures are 

 seen to pass backward, forming long and narrow cores. These are evidently of 

 hard, bony texture, for they often stand out from the plate-like ridges when 

 the intervening basal portion of the plate is worn away. We also observe that the 

 adjacent mucous membrane of the roof, sides, and floor of the mouth is studded 

 with distinct papillae. These, it will be seen, correspond to the "tritors," in 

 size, prominence, and closeness in arrangement, and may, I believe, from the evi- 

 dence of similar structures in the mouth region of various fishes, be looked upon as 

 homologous with tooth-forming papilla;.* It will thus be observed, as in figs. 94 a, 

 94 B, that they occur within the stomadeal region; they are absent in the dorsal 

 wall of the pharynx; they are present, however, on the floor of the mouth, and are 



*In a recently published paper on the oral and pharyngeal denticles of elasmobranchs (Proc. Zool. Soc, 1905, I, 

 pp. 41-49), Imms gives reasons for homologizing similar structures in sharks with teeth. He did not, however, find 

 the papillae present in the specimen of Chimccra moiislrosa which he examined. 



