366 ANNALS NEW YORK ACADEMY OF SCIENCES 



transformation has occurred many times in the Actinopterygii and also 

 in the Dipnoi. If the pro-Tetrapoda were Rhipidistia the transformation 

 also inTolred the loss of the two dorsal and anal fins and of their ex- 

 panded basal supports. The notochord in these pro-Tetrapoda mnst have 

 been wholly persistent, for in the earliest known tetrajDods the vertebrae 

 are not only of widely varied type, but by no means complete. The pro- 

 Tetrapoda undoubtedly had both the pectoral and pelvic fins strongly 

 developed and of lobate or fleshy-based type. When they first emerged 

 from the water, either in pursuit of littoral prey or during times of 

 drought, they may have used the medial or palmar surface of the paddles 

 for traction and propulsion, at the same time wriggling on their bellies. 

 Some of those with small paddles soon adopted a snake-like habitus and 

 lost the limbs (Aistopoda, Apoda). In those that developed the limbs 

 (Fig. 14) the scapulocoracoid increased rapidly in size and became ossi- 

 fied, the cleithrum was reduced, the posttemporal and supracleithrum 

 disappeared and the only connection between skull and shoulder-girdle 

 was furnished by the "trapezius'^ muscle, which in modern urodeles ex- 

 tends from the "suprascapula" ( ? cleithrum) to the occiput (Fig. 14, F). 

 Thus the shoulder-girdle acquired mobility, while, with the growth of the 

 scapulocoracoid, its muscles acquired a larger base, and extended around 

 from the medial to the external side. The extension of the coracoscapular 

 muscles, ventrally, dorsally and externally, together with the differentia- 

 tion of the pectoral muscles greatly increased the strength of the forearm 

 and crowded the cleithrum to the front edge of the scapula. The differ- 

 entiation of the pectoral muscles conditioned the formation of the rhom- 

 boid interclavicle which partly overlies them. 



More difficult to comprehend are the muscular readjustments which 

 must have ensued when the elbow and knee bends were being established, 

 and the shifting of the articular surfaces of the humerus radius and ulna. 



With regard to the muscles of the cheiropterygium itself. Professor 

 H. H. Wilder (1909, pp. 230-231, 235) in the course of an illuminating 

 discussion of the musculature of Necturus writes as follows : 



"The muscles of the distal portion of the vertebrate chiropterygium, that is, 

 from elbow or knee on, aside from the modifications imposed upon them by 

 the varying shapes of the limbs themselves, and the great difference in their 

 use, are, in their essential features, quite similar in all living forms, and in 

 their differences show the modifications of a primary tj'pe due to en\i.ronment 

 rather than the suggestions of an historic development of that type. The 

 study is, therefore, one mainly of the adaptations of a given set of elements, 

 rather than a phylogenetic history, which latter, as is the case also with the 

 bones of the same region, must be sought in the gap separating fin and hand, 

 that is, in the phylogenetic stages represented by lost forms of ganoids, stego- 



