675 



Fig. 1. 



Fi2 2. 



>^^ 



Fig. 3. 



h>.. 



Fig. 4. 



Fig. 1. Hypothetical condition of the pectoral fin in the ancestr»! CUdoselachid. 

 Fig. 2. Hypothetical condition of the ventral in the ancestral Cladoselachid. 

 Fig. 3. Pectoral, and 



Fig. 4 ventral fin of Cl&doselacbe ^ple^i Xi- -Sb»salia;Z) dermal mar- 

 gin of fin: B radialia. 



These appear to be attached at the body wall to a series of separate 

 basalia, B. It is especially noteworthy that in the shortening of 

 the fin's length, the radial supports have become crowded in the fore- 

 and mid-region of the fin, and that a tendency is evident to inter- 

 calate the tips of the radials. It may, I think, be reasonably inferred 

 that in its ancestral condition this ventral fin must have been some- 

 what longer, perhaps not quite so wide, with a less marked tendency 

 toward the crowding of radials and basals. These conditions have been 

 hypothetically fulfilled in the accompanying figure, Fig. 2. 



That the structure of the pectoral fins is to be derivated from 

 the simpler conditions of the ventrals has been ably demonstrated 

 by WiEDERSHEiM. It is not, accordingly, surprising that the pectorals 

 of some of the Cladoselachids correspond clearly to the ventrals of 

 others. In Fig. 5 is shown the pectoral of C. newberryi which 



44* 



