238 



COMPARATIVE ANATOMY 



between the dorsal extensor muscles of the appendage and the ventral 

 flexor muscles. 



Further steps in the evolution of the appendicular skeleton involve 

 the thickening and fusion of the basal or proximal portions of the radial 

 cartilages and the extension of the basal cartilages thus formed into the 

 body wall and towards the mid-ventral line. The result of this appears 

 today in the pelvic fin of elasmobranchs. The beginnings of a girdle are 

 seen in a ventral cartilaginous plate, the ischio-pubis. A doubtful 

 beginning of the ilium may be seen in the so-called iliac process. The 



PREAXIAL BORDER TO RIGHT 



ABCD HYPOTHETICAL STAGES 



Fig. 194. — Diagrams illustrating the hypothetical evolution of the extremities of 

 Dipnoan (7), Ganoid (//), and Elasmobranch (G) from a fin-fold supported by a series 

 of similar radial cartilages. By fusion basal elements are differentiated. The skeletal 

 supports of fins eventually differ in the relations of the basal elements to the radialia. 

 (Redrawn after A. Brazier Howell.) 



evolution in the pectoral girdle seems to have been more rapid than in the 

 pelvic girdle, if we may base our conclusion on the fact that in elasmo- 

 branchs the scapula or dorsal arm of the pectoral girdle is already well 

 developed when there is little, if any, indication of a dorsal arm, the ilium, 

 of the pelvic girdle. In both girdles in the elasmobranch, however, 

 a ball-and-socket articulation between girdle and free extremity has 

 already made its appearance. 



An advance towards the pectoral girdle of higher vertebrates appears 

 in living and fossil ganoids in which a membrane bone, the clavicle, is 

 added to the pectoral girdle. There is no structure in the pelvic girdle 

 homologous with the clavicle. 



A tripartite pectoral girdle makes its first appearance in amphibia. 

 The ventral arm, which in fishes was single and undivided, becomes in 



