658 MR. F. M. BALFOUR ON THE SKELETON [June 7, 



angles with the long axis of the body, were developed in varying 

 extents in all these four longitudinal folds. 



"3. The longitudinal folds became interrupted variously, but so 

 as to form two prominences on each side, i. e. the primitive paired 

 limbs. 



"4. Each anterior paired limb increased in size more rapidly than 

 the posterior limb. 



"5. The bases of the cartilaginous supports coalesced as was 

 needed, according to the respective practical needs of the different 

 separate portions of the longitudinal folds, i. e. the respective needs 

 of the several fins. 



" 6. Occasionally the dorsal radials coalesced (as in JSTotidanus 

 &c.) and sought centripetally {Pristis &c.) adherence to the skeletal 

 axis. 



" 7. The radials of the hinder paired limb did so more constantly, 

 and ultimately prolonged themselves inwards by mesiad growth from 

 their coalesced base, till the piscine pelvic structure arose, as, e.g., in 

 Squatina. 



" 8. The pectoral radials with increasing development also coalesced 

 proximally, and thence prolonging themselves inwards to seek a 

 point d'appui, shot dorsad and ventrad to obtain a firm support, 

 and at the same time to avoid the visceral cavity. 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 pre- 

 served and enlarged preexisting cartilages or developed fresh ones to 

 serve fresh needs, and so grew into the developed cheiropterygium ; 

 but there is not yet enough evidence to deteriuine what was the 

 precise cQurse of this transformation. 



"11. The pelvic limb acquired a solid connexion 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 structure 

 became also quite similar (e. g. Ichthyosaurus, Plesiosaurus, Chelydra, 

 &c.) ; but for the ordinary quadrupedal 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." 



Giinther ' 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 Giinther's views on the nature of the skeleton of the fins more 

 especially depends upon the fact that he attempts to evolve the fin 



' "Description of Ceratodus," Phil. Trams. 1871. 



