. Chordate anatomy. Chordata; Anatomy, Comparative. 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
. Chordate anatomy. Chordata; Anatomy, Comparative. 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 p
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