COMPARISON OF DENTAL PLATES. 123 



Comparing now a series of the dental plates of Chimseroids (figs. 95 to 103), 

 we may first place side by side those of Callorhynchus and Harriotta (figs. 95, 96). 

 It then becomes clear, I think, that the ridges in the dental plates of the former 

 genus correspond to the clustered tubercles in Harriotta, a comparison which is 

 well borne out by the embryological studies of Schauinsland, for it will be recalled 

 that the separate ridges of Callorhynchus were shown to consist of a mass of chalky 

 centers in which the lamellae of dentine were parallel to one another, although their 

 substance, as was noted, remains uncalcified (^. iig. 105). A similar state of affairs, 

 it may be remarked, occurs in the posterior part of the large tumid ridges in 

 Harriotta, for these ridges and their tritors can be readily sectioned. On the other 

 hand, the anterior eminences of the same tumid ridges are found to be much harder 

 than the neighboring bony plate, and may with less question, therefore, be regarded 

 as representing true teeth. Indeed, it is, after all, a matter of minor importance 

 that these tritoral elements have never hardened in the case of Callorhjmchus; for 

 when we consider the thickness and hardness of the surrounding bony plate, we 

 are led to conclude that this may well have usurped the function of the separate 

 denticles, and that these therefore remain undeveloped. The same rudimentary 

 condition is probably true of the minute tritoral points which one finds along the 

 anterior margin of the vomerine plates in Callorhynchus. 



Continuing the comparison, one can with fair definiteness understand the 

 relations between the dental plates of such forms as Harriotta and Cliinicera 

 phantasvia. For, in the latter, the wide tritors at the base of the palatine and 

 mandibular plates (fig. 97) are evidently homologous with the clustered tubercles 

 in Harriotta. In C. phantasnia, however, the crushing surfaces of the plate are 

 smoother and less extended. In C. mediterranea (fig. 98) the dental plates have 

 become more oblique (slanting) in their manner of attachment, the posterior flange 

 of the plates intruding deeply below the mucous fold in the roof of the mouth. In 

 C. tjioiistrosa (fig. 99) the tritoral areas of the palatine plates are less numerous, 

 while in the mandibular plates they are more abundant, but show less clearly 

 the peculiar banded structure of the foregoing specimen. In C. viitsukurii 

 (fig. 100) the conditions are not widely different from those in the species from the 

 Mediterranean. A peculiar arching appears in the palatine plates, and the ridges 

 on the posterior face of the mandibular plates, although smaller, are more con- 

 spicuous. In C. aflinis (fig. 10 1) the proximal tritoral areas were not observed, and 

 altogether the grinding margin of the palatine and mandibular plates was narrower. 

 In C. collici (fig. 102), while the tritoral ridges on the posterior faces of the pala- 

 tine and mandibular plates are (usuall}') conspicuous, the grinding edges of these 

 plates are exceedingly narrow. And in Rhinochimcera pacifica (fig. 103), finally, 

 we attain a condition, as we have already noted, in which the tritoral areas are 

 reduced to obsolescence, the entire distal margin of the plate functioning as a 

 cutting edge. 



From what has already been said regarding the dental plates in C. colliei 

 (p. 19), I think we may safely conclude that a wide range of variation occurs in the 

 dental plates of Chimaeroids. Thus the tritoral structures may vary in number, size, 



