thp: skeletal system 



179 



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 elasmobranchs 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. 



PREAXIAL BORDER TO RIGHT 



ABCD HYPOTHETICAL STAGES 



Fig. 168. — Diagrams illustrating the hypothetical evolution of the extremities of 

 dipnoan (/), ganoid (H), 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.) 



A tripartite pectoral girdle makes its first appearance in amphibians. 

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

 amphibians differentiated into posterior and anterior moieties, the coracoid 

 and precoracoid. The dermal clavicle becomes closely apposed to the 

 precoracoid . The dorsal scapula and suprascapula remain undivided as in 

 fishes. The dorsal arm of the pelvic girdle, the ilium, articulates with the 

 transverse process of a single sacral vertebra. In its most primitive form in 

 amphibians, the ventral portion of the pelvic girdle resembles that of ganoid 

 fishes, and consists of a broad cartilaginous plate with which the femur 

 articulates. See Fig. 169. Centers of ossification corresponding with the 

 ischium and pubis of reptiles arise successively in this plate. 



