Ilg CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



clasping organ is usually depressed. In this connection we call to mind the great size 

 of the clasping organ in the young Callorhynchus, suggesting its origin from an 

 anterior fin spine (cf. figs. 132-137); its small size in Chimaera on the other hand 

 indicates the later derivation of this genus. 



This induction is also supported by a study of the clasping organs connected 

 with the ventral fins in the antero-pelvic clasping organ of Callorhynchus. We 

 observe that this structure is furnished with many dermal denticles 40 or there- 

 abouts in the case of Callorhynchus antarcticus , according to Dumeril, whereas in 

 the various species of Chimaera and in Rhinochimaera the number is reduced, varying 

 usually from about six to three. 



In the mixipterygium shagreen denticles occur plentifully. In the case of one 

 arm of this trifid organ in Chimcera collici the denticles extend proximally as far as 

 the base of the organ. In the other two arms the shagreen is limited to tracts 

 near the tips. An abundant supply of these denticles is, however, present, repre- 

 senting, in fact, tracts of shagreen. In Chimcera monstrosa, on the other hand, the 

 amount of the shagreen is less, a condition which furnishes another reason for 

 regarding this species as the more modified. In Rhinochimcera pacifica, as the 

 writer has already noted (Jour. Sci. Coll. Tokyo, vol. xix, p. 10), the shagreen at 

 the tip of the mixipterygium is greatly reduced. In Callorhynchus, on the .other 

 hand, it is as abundant as in the case of Chimcera colliei. 



DENTAL PLATES. 



These have always been the stumbling-block in comparing Chimaeroid with 

 sharks, for by only superficial comparison have the tritoral areas in the dental 

 plates of Chimseroids been regarded as equivalent to the teeth or clusters of teeth 

 in the shark. Nor has paleontology as yet been able to elucidate the problem, 

 even to the degree in which it has thrown light upon the origin of the dental plates 

 in the lung-fishes. In fact, as we shall later note, the study of the dentition of 

 fossil Chimaeroids leads us at the present time to no decisive results. The develop- 

 ment of the dental plates might therefore be looked to to furnish evidence as to the 

 nature of these structures. For it is well known that through embryolog}' a flood of 

 light has been thrown upon the mode of origin of the dentition of lung fishes. 

 Accordingly, we conclude that one of the most important sections of Schauinsland's 

 memoir on Callorhynchus is devoted to the question of the mode of origin of the 

 dental plates. 



Schauinsland's account, indeed, is of such value in the present connection that 

 I have been led to quote it in freely translated form (pp. cit., pp. 13-16) : 



In even their earliest stages the dental plates are laid down as distinct elements, i. e., four 

 above and two below, and there is at no time a definite indication that these are composed of 

 simpler elements which have fused together. The upper anterior plates are certainly simple ; 

 the remaining pairs, however, show along their hinder (caudal) border a somewhat trifid 

 arrangement. In this region, too, the plates with their three ridges pass into a fold of the skin, 

 and here their growth takes place. (No trace appears even in earlier stages of the median 

 (unpaired) mandibular tooth which has been described in fossil Chimaeroids.) If we regard the 

 three ridges as rows of teeth which have become fused together, they would have obviously a 

 certain similarity to the dental plates of dipnoi or even of teleosts (e. g., Anarrhichas); and we 



