DENTAL PLATES OF LARVAL CHIM.«:rOIDS. 



125 



as conspicuous, is none the less suggestive when we compare it with the strictly 

 tritoral conditions shown in Harriotta, fig. 94 a. 



DENTAL PLATES OF LARVAL CHIM.EROIDS. 



Furthermore, if one compares the dental plates in Chima^roids of different 

 stages of growth, one is impressed with the evidence of larval adaptations. The 

 plates of a Chimeeroid recently hatched (C colliei) are surprisingly large in size, 

 but instead of spreading out in lOJ 

 the form of crushing plates, they 

 protrude marginally, forming 

 relatively high edges and 

 function evidently in cutting. 

 Moreover, the substance of these 

 juvenile plates is glassy {c1 . 

 Schauinsland, re vitrodentine) 

 rather than horn-like or chalky, 

 and their margins are sharp and 

 brittle. It is clear, therefore, 

 that the plates grow during 

 earlier stages, notably at their 

 outer or secant margins, and it 

 is a probable conclusion that 

 this condition of growth is corre- 

 lated with the special feeding 

 requirements of the young. In 

 later stages the plates broaden 

 and thicken, the secant edges 

 become less and less conspicu- 



j J 11 4-1, «- 4- 1 F'g5- 107-109. — HarrioUa raleighana. Dental plates (somewhat diagrammatic) 



OUS, and gradually the tritoral ^, ^^^^^ individuals measuring respecHvely 10, 49, and 64 cm. At A the 



areas appear. ihe latter, at lateral aspect of the vomerine and palatine plates is given. 



least in the species examined, are developed first vaguely, in extended tracts or 

 rido-es, and in these there later arise discrete eminences. This is the condition 

 indicated above in Callorhynchus {cf. also with fig. 95 the juvenile plates shown in 

 fig. 106); it is even more marked in CltiuuFra colliei, and it is to be observed in 

 such a form as Harriotta. Of the last form we may introduce sketches of three 

 stages of the dental plates.* In the first (fig. 107) the plates are frail, although 

 well formed, and with secant prosilient edges; they have already ridges outlined 

 and their clouded color (especially in the palatines) is probably due to the presence 

 of vitrodentine. In the second stage (fig. 108) a number of distinct tritoral emi- 

 nences appear. And in the final stage (fig. 109), the largest specimen of Harriotta 

 recorded, the tritors are well dift'erentiated. 



*For the privilege of examining this unique material the writer is indebted to the United States National Museum. 

 He wishes especially to express his thanks to its Assistant Secretary, Mr. Richard Rathbun, and to its assistant 

 curator of Fishes, Mr. Barton A. Bean. 



