THE SKELETAL SYSTEM 



243 



disagree as to the skeleton of the primitive extremity, the archipterygium. 

 While some suppose it to have been uniserial, i.e., the radial cartilages 

 were limited to one side of the basal cartilages or axis as in elasmobranchs, 

 other morphologists regard the biserial fin skeleton of dipnoi as the more 

 primitive. Conclusions in regard to the evolution of the skeleton of the 

 extremity differ, therefore, as one or other of these two types of fish-fin 

 skeleton is assumed as more primitive. 



PRE AXIAL 

 DIGIT 



Fig. 202. — Goodrich's solution of the problem of the transformation of the ichthy- 

 opterygium into the Cheiropterygium. The diagram (B) of the ichthyopterygium is 

 based upon the fossil fish Sauripterus. The main axis of the appendages is indicated 

 by a dotted line. A and C are diagrams of cross sections showing the relations of girdle 

 and extremity. D represents the appendicular skeleton of Tetrapods. (Redrawn after 

 Goodrich.) 



There is also extreme divergence of opinion as to the exact homologies 

 between the elements of the ichthyopterygium and of the cheiropterygium. 

 The skeleton of the latter invariably articulates in a socket of the girdle 

 by a single skeletal element, the humerus or the femur. The fish extrem- 

 ity, on the contrary; usually articulates by more than a single skeletal 

 element. While the forearm and shank of tetrapods have respectively 

 two elements, radius and ulna and tibia and fibula, the number in the 

 ichthyopterygium is usually considerably greater than this. Obviously, 

 in the evolution of the skeleton of the fish extremity into that of land 

 animals there has been a loss of skeletal elements as well as a great elonga- 

 tion of the appendage. It is difficult to determine which elements have 

 persisted and which have disappeared. See Fig. 200. 



