. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Acromion ^u„A-»Clavicle w\ .MT Coracoid-^-i/_£/ N Interclavicle' Sft^VW Fig. 550. Left-lateral views of the pectoral girdles of three primitive reptiles. A, a cotylosaur, Diadectes; b, a cotylosaur, Labidosaurus; c, a therapsid, Dicynodon. (After Romer.). investing interclavicle and clavicles but never cleithra or procoracoids. The pectoral girdle of turtles consists on either side of a tripod of rodlike bones which lacks the clavicle. The dorsally extendi
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Acromion ^u„A-»Clavicle w\ .MT Coracoid-^-i/_£/ N Interclavicle' Sft^VW Fig. 550. Left-lateral views of the pectoral girdles of three primitive reptiles. A, a cotylosaur, Diadectes; b, a cotylosaur, Labidosaurus; c, a therapsid, Dicynodon. (After Romer.). investing interclavicle and clavicles but never cleithra or procoracoids. The pectoral girdle of turtles consists on either side of a tripod of rodlike bones which lacks the clavicle. The dorsally extending part of the tripod is the scapula, and the posterior ventral element, the coracoid, but the anterior ventral element, which was formerly taken to be the investing clavicle, is now known to be an outgrowth of the replacing scapula and is termed the acromion. It is homologous to the acromion process of the mammalian scapula (Fig. 551) with which the clavicle articulates. The interclavicle and the clavicles are now known to participate in the formation of the plastron, the ventral part of the turtle's shell. The pectoral girdle of turtles is peculiar in another way. The whole structure, as well as the pelvic girdle, is inside the ribs which form part of the shell, an ar- rangement not found among other vertebrates. In the embryos the girdles are still outside the ribs as usual, but with the modification of the ribs into costal plates that flatten out and unite edge to edge to contribute to the carapace, or dorsal part of the shell, the two girdles sink in and are covered over by the expanding ribs. In crocodiles the clavicle, although appearing in the embryo, does not persist in the adult. The interclavicle is a simple rod in crocodiles (Fig. 464) but T-shaped in lizards (Fig. 463). The coracoid and scapula of lizards are frequently fenestrated, that is, broken up by window-like parts. Pectoral girdles are missing in snakes and much reduced in the legless lizards. Fig. 551.
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Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
