OF THE PAIRED FINS OF ELASMOBRANCHS. 717 



Thus they came to abut dorsally against the axial skeleton, and 

 to meet ventrally together in the middle line below. 



" 9. The lateral fins, as they were applied to support the body 

 on the ground, became elongated, segmented, and narrowed, so 

 that probably the line of the propterygium, or possibly that of 

 the mesopterygium, became the cheiropterygial axis. 



" 10. The distal end of the incipient cheiropterygium either 

 preserved and enlarged preexisting cartilages or developed fresh 

 ones to serve fresh needs, and so grew into the developed cheir- 

 opterygium ; but there is not yet enough evidence to determine 

 what was the precise course of this transformation. 



"ii. The pelvic limb acquired a solid connection with the 

 axial skeleton (a pelvic girdle) through its need of a point 

 d'appui as a locomotive organ on land. 



" 12. The pelvic limb became also elongated ; and when its 

 function was quite similar to that of the pectoral limb, its struc- 

 ture became also quite similar (e.g. Ichthyosaurus, Plcsiosaurus, 

 Chelydra, &c.) ; but for the ordinary quadrupedal mode of pro- 

 gression it became segmented and inflected in a way generally 

 parallel with, but (from its mode of use) in part inversely to, the 

 inflections of the pectoral limb." 



Gunther 1 has propounded a theory on the primitive character 

 of the fins, which, on the whole, fits in with the view that the 

 paired fins are structures of the same nature as the unpaired 

 fins. The interest of Glmther's views on the nature of the 

 skeleton of the fins more especially depends upon the fact that 

 he attempts to evolve the fin of Ccmtodus from the typical Sela- 

 chian type of pectoral fin. His own statement on this subject 

 is as follows 2 : 



" On further inquiry into the more distant relations of the 

 Ceratodns-\\\rib, we may perhaps be justified in recognizing in it 

 a modification of the typical form of the Selachian pectoral fin. 

 Leaving aside the usual treble division of the carpal cartilage 

 (which, indeed, is sometimes simple), we find that this shovel- 

 like carpal forms the base for a great number of phalanges, 

 which are arranged in more or less regular transverse rows (zones) 

 and in longitudinal rows (series). The number of phalanges of 



1 " Description of Ceratodus" Phil. Trans. 1871. 

 3 Loc. cit. p. 534. 



