STRUCTURE OF THE VERTEBRATES 



159 



specialized members of this order had a bony skeleton which is 

 clearly a pre-amphibian arrangement. The middle bone was 

 large, forming the main support of the fleshy lobe (page 44), 

 and distally were two other bones. These three bones are homolo- 

 gized with the proximal humerus and the radius and ulna of the 

 forearm. Distal to these were small bones which evidently gave 

 rise to the carpals of the wrist joint, and the metacarpals and 



Shoulder girdle 



A. Dogfish 



B. Primitive Extinct 

 Crossopterygian 



Fig. 95. Stages in the Evolution of the Pectoral Fin. Compare the Dogfish 

 fin with that of Sauripterus (Order Crossopterygii) . The latter has all the 

 elements of a form ancestral to the amphibia. (Sauripterus after Hussakoff). 



bones of the digits. Two genera of these fish have been care- 

 fully studied, Sauripterus and Eusthenopteron, the latter being 

 illustrated. 



The girdle of these fish had become ossified as two bones, the 

 ventral coracoid and the dorsal scapula, which had kept their 

 primitive positions. In addition to these replacement or cartilage 

 bones, three small dermal bones had become attached to the 

 scapula at its dorsal margin. These formed an attachment be- 

 tween the girdle and the head. Another pair of bones was added 



