FINS OF ELASMOBRANCHS. 449 



CESTRACION PHILIPPI. 

 Pectoral Fin (Plate LXXVI. fig. 6). 



In this form ^ there are but two basal cartilages Id the pectoral fin, one of which, the 

 undoubted metapterygium, is in the form of an elongated triangle, with the apex 

 proximad. It supports about ten radials, pairs of which may or may not coalesce 

 together proximally. 



The other basal cartilage, apparently answering to both fro- and mesopterygium, is a 

 large rhomboidal plate, with its narrowest border proximad, and supports the rest of 

 the radials, which appear to be about seven in number. 



The bases of these radials, however, have either coalesced into, or ha^e connately 

 arisen as, thin, large, irregularly formed, but rather oval plates, two of which adjoin the 

 distal margin of the pro-mesopterygium, which margin they entirely occupy except 

 towards its postaxial end, where it gives attachment to five uncoalesced radials. 



The third large oval plate is attached to the distal margin of the more postaxiad of 

 the other two oval plates, and to the postaxiad margin of the more preaxiad one, 

 extending distad beyond the latter. The two preaxial oval plates, thus succeeding each 

 other distad end to end, form the preaxial margin of the limb-skeleton, save for one 

 or two very small pieces of cartilage, which are continued on distad by the most pre- 

 axial of the radials. 



This exceptional form serves to demonstrate how radials may coalesce (or arise con- 

 nately) at their proximal ends, so as to be replaced by large cartilaginous plates. We 

 may, if we please, call these plates segmented pro- and mesopterygia ; but there seems 

 no good reason for doing so, since they appear to represent merely coalesced radials. 

 Rather, I venture to think, they suggest the probability that the pro-, meso-, and me- 

 tapterygium may themselves be nothing but plates formed by the coalescence of the 

 bases of radials which were primitively distinct. 



The whole limb-skeleton extends distad to a much greater extent preaxially than 

 postaxially, in harmony with the external form of the whole fin. 



ACANTHIAS BLAINVILLII. 



Dorsal Fin (Plate LXXVII. fig. 1). 



In this fin the skeleton may be considered as formed of the normal three super- 

 imposed longitudinal series of adjacent cartilages, specially modified with respect to 

 the sharp spine which it bears at its preaxial end. 



The basal cartilages are represented by the cartilaginous base of the spine, by one 



' Represented by Prof. Huxley in the P. Z. S. 1876, p. 51 , fig. 1 1 ; and by Prof. Gegenbaur, ' UnterBuchungen,' 

 Heft 2, plate ix. fig. 3. 



3p2 



