54 MR. r. M. BALFOUR ON THE SKELETON 



developed cheiropterygium ; but there is not yet enough evi- 

 dence to determine what was the precise course of this trans- 

 formation. 



*^ 11. The pelvic limb acquired a solid connection with the 

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

 (Tappui 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 structure became also quite similar {e.g. Ichthyosaurus, 

 Plesiosaurus, Chelydra, &c.) ; but for the ordinary quadru- 

 pedal mode of progression 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." 



Glinther^ has propounded a theory on the primitive cha- 

 racter 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 tins. The interest of Giinther^s views on the nature 

 of the skeleton of the fins more especially depends upon the 

 lact that he attempts to evolve the fin of Ceratodus from the 

 typical Selachian type of pectoral fin. His own statement on 

 this subject is as follows :^ 



^'On further inquiry into the more distant relations of the 

 Ceratodus-\\mh 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 trans- 

 verse rows (zones) and in longitudinal rows (series). The 

 number of phalanges of the zones and series varies according 

 to the species and the form of the fin ; in Cestracion philippi 

 the greater number of phalanges is found in the proximal 

 zones and middle series, all the phalanges decreasing in size 

 from the base of the fin towards the margins. In a Sela- 

 chian with a long, pointed, scythe-shaped, pectoral fin, like 

 that of Ceratodus, we may, from analogy, presume that the 

 arrangement of the cartilages might be somewhat like that 

 shown in the accompanying diagram, which I have divided 

 into nine zones and fifteen series. 



" When we now detach the outermost phalanx from each 

 side of the first horizontal zone, and with it the other 

 phalanges of the same series, when we allow the remaining 

 phalanges of this zone to coalesce into one piece (as, in 

 nature, we find coalesced the carpels of Ceratodus and many 



' " Description of Ceratodus," ' Phil. Trans./ 1871. 

 ' Loc. cit., p. 534. 



