126 CHIM.-EROID FISHES AND THEIR DEVELOPMENT. 



From the foregoing characters in "larval" dental plates, and they are certainly 

 in the general line of Garman's observations,* we conclude that among the many 

 specializations in the young Chimseroid may be included a larval dentition, /. e., 

 preceding the appearance of tritors. It may also be remarked that the tritors 

 themselves, when they come to appear in the different forms of Chimseroids, occur 

 in point of time in interesting sequence. In Callorhynchus they appear in the 

 embrj^o (95 mm.), while it is still encapsuled, but they fail to develop into typical 

 structures; in their place there appear calcified ridges representing collections of 

 tritors. In Harriotta tritors become functional at a period shortly after hatching, 

 and from this time onward increase both in size and number. In Chimaera they 

 occur at a later period, develop slowly, and even in the adult are relatively few, and 

 the plates themselves early develop secant margins. In Rhinochim?era, finall}', 

 they appear only in the adult, and even then in rudimentary form. In the Chim- 

 aeroid series, there is thus, I think, such evidence of progression, even in recent 

 forms, that we can hardly assume with Garman that from a condition like that in 

 Rhinochimaera arose the dental plates of the other genera. On the contrary, in the 

 case of Rhinochimaera we are dealing evidentlj' with a terminal form, one in which 

 the tritors fail to develop perfectly even in the adult, f 



CONCLUSIONS CONCERNING THE DENTAL PLATES OF RECENT CHIM.*;rOIDS. 



A comparison of a series of the dental plates of recent Chimseroids, as we 

 have seen, strengthens the view that these structures are compound, /. c. , formed of 

 separate denticle-like elements, homologous with the dental plates of certain sharks, 

 e. g., Cestracionts. The tritors, according to this view, represent dental eminences, 

 simple or compound. But more doubtful is the homologue of the dental plate 

 itself. It may represent either the fused bases of teeth like the Cestraciont, or 

 a structure entirely sui generis, i. c, fused by a hardening of the connective tissue 

 accumulated around the bases of the true dental plates. According to the observa- 

 tions of Schauinsland the embryological facts support more or less distinctly the 

 origin of the tritoral ridges from many tooth-like eminences dentinal in structure. 

 On the other hand, the same evidence tends to regard the substance of the dental 

 plate itself as independent of the tritors. An examination of the larval dentition of 

 Chimseroids throws, I think, a side-light on the foregoing discrepancy, for it is found 



*Garman, however, interprets these characters (Proc. New Eng. Zool. Club, 1901, vol. 11, pp. 75-76) not as larval- 

 isms, but as primitive ; thus, according to him " the teeth of Rhinochimaera are of a much less difierentiated form than 

 those of any other of the recent genera of the group ; that is, their later stages are more like the earlier, and presum- 

 ably more like the teeth of primitive Chimsroids ; they approach those of the extinct myriacanths and the very early 

 conditions of the teeth of other living Chimseroids, Chimsera, Callorhynchus, and Harriotta. In advanced stages, the 

 teeth of Harriotta differ from those of Rhinochim^ra in possessing several series of tritors which in superficial aspect 

 resemble, in shapes and arrangement, certain crowns of placodont teeth. On the teeth of Rhinochimaera there are no 

 tritors ; the teeth of the very young of the other living genera are similar ; this no doubt is a mutual resemblance to 

 those of a common ancestor, an index to derivation. * * * " To this interpretation, on the other hand, there are 

 two somewhat critical objections : (i) that in Rhinochim;era, as this author has later observed, there are present tritoral 

 points, small, it is true, but tritors none the less ; and (2) that his conception of the dental plates of fossil Chimaeroids 

 (c g^., Myriacanth) is not valid, for whatever be the puzzles of the dental plates of fossil Chimseroids they have always 

 tritoral areas. 



tThey may be expected to appear in a more perfect condition in very old individuals, somewhat as they develop in 

 the late rather than in the young larva" of Chimaera. 



