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CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



young sharks, save that, as in the case of many other chimseroid structures, the rate 

 of growth is accelerated; the lateral-fold beginnings extend over fewer body segments 

 and are higher (proximo-distally), leading us to conclude that in this mode of 

 early fin growth the Chimaeroid exhibits the same relation to the shark that the tel- 

 eost bears to the ganoid. Especially convincing evidence as to the modified nature 

 of the chimseroid fin is produced by the development of the ventral "claspers"; for 

 these, the antero-ventral hooks and the mixipterygia, are to be regarded as highly 

 modified radials. The antero-ventral clasper, it is clear, has not yet been evolved 

 in the sharks, unless the greatly enlarged anterior lappet of the ventral fin be 

 regarded as its equivalent ; but there is good foundation for the belief that in 

 Chimseroids between the antero-ventral organ and the compressed lappet of the 



B 



Fig. 1 1 2. Ventral fin and appendages in Chimaera colliei. 



A, Fin of young specimen (3 1 cm. in length) ; ventral aspect showing mixipterygia and antero-ventral clasper, the latter still connected by dermal crease with the 

 anterior rim of fin ; c, mixipterygium with lips unfolded ; B, skeleton of foregoing fin, showing the arrangement of the supports (radials) of the branches of 

 the mixipterygium : C, skeleton of fin, adult: D, skeleton of ventral fm of Cestracion (Heterodontus japonicus), adult, for comparison with foregoing. 



pelvic fin there formerly existed a number of radialia ; witness, for example, the 

 rudiments of the segmentation of the basal plate from which the antero-ventral 

 organ arises (fig. 112, nerve and vessel openings in B and c),f or better still, the 

 radials which persist in the anterior reach of the fin of the Jurassic Chimgeroid, 

 Squaloraja (fig. 138, ar). The mixipterygium also bears testimony to having 

 been closely connected with the radials of the base of the fin ; thus in one stage in 

 development, cf. fig. 112 B, the base of the mixipterygium bears rudiments of 

 radialia, and the trifid tip is in itself a relic of a clustering of distal radials. These 

 observations are clearly in line with Jungersen's, who, while admitting that the 

 ' ' appendix-skeleton of the Holocephales is of less compound construction than that 

 of Plagiostomes," calls attention to the "wide separation of the whole organ (i. e.,} 



*In the adult Chimseroid the basal articular element of the pectoral fin is usually termed (as in Cestracion) 

 mesopterygium, and it is regarded (Gegenbaur, 1901) as including also the propterygium ; Schauinsland, however, 

 has shown (op. cit., Taf. xxiv, fig. 174) that the bibasal character of the fin is due to the obsolescence of the 

 metapterygium. The articular basal is, therefore, the propterygium. With this result the present writer is in accord. 



fThis conclusion was originally suggested by Gegenbaur (1901) on the evidence of adult anatomy. 



