468 



PEOF. ST. GEORGE MIVAET ON THE 



He arrives at the conclusion that the paired and azygos fins are of similar nature, 

 and are " specializations of two lateral folds, as the dorsal and anal fins were specializa- 

 tions of the median folds." He has, moreover, the merit of having arrived at this con- 

 clusion before becoming acquainted with the mode of development of the Elasmobranch 

 paired fins described by Mr. Balfour. 



The important point on which Mr. Thacher has gone beyond me is in the suggestion 



that not only have the paired fins themselves been developed centripetally, but that the 



pelvic girdle itself is but due to the further extension inwards of that centripetal 



growth : — 



Fig. 2. 



—a 



^' 



^■^:2 



D. 



Diagram illustrating Mr. Thaoher's conception of the genesis of the pelvic girdle. 



A. Left- ventral-fin skeleton seen from beneath in its primitive condition. 



B. The same, with the proximal ends of the radials coalesced into a longitudinal basal cartilage. 



C. The same with the basal cartilage segmented. 



D. Both ventral fins, showing the development of a median process from the preaxial basal cartilage of 



E. The same, with the preaxial cartilages medianly adherent, and the proximal part of the preaxial 



coalescing. 



each, 

 radials 



I regard this as a most happy suggestion, and I give it my entire adhesion. 



Mr. Thacher says little ^ about the nature or formation of the shoulder-girdle, and 

 nothing as to the causes of its mode of attachment to the skeletal axis and of the 

 manner of its growth inwards from tlie pectoral fins. But the very difl'erent conditions 

 of the two limb-girdles in Fishes seems to me to readily explain these phenomena. 



The more powerful action of the pectoral fins, as compared with the ventrals, requires 

 for them a firmer ^JO»i# d'appui than is needed for the latter. The ingrowth from them, 

 therefore, seeks a direct solid fixation, just as in Pristis and Pristiophorus the resistance 

 to lateral pressure, in using their saw, is facilitated by the direct solid fixation of the 

 dorsal fin to the skeletal axis. But the reasons before enumerated ^ as hindering the 

 direct ingrowth of the pectoral-fin base, or the formation of an antero-posterior longi- 

 tudinal connexion between it and the spinal column, determine both the shape and 



' His words are : — " The cartUages, spreading, met in the middle line ; and a later extension of the cartilages 

 dorsad completed the Umb-girdle." — L. c. p. 298. " See anU, pp. 465, 466. 



