. The biology of the amphibia. Amphibians. THE SKELETON 237 pitidae, both clavicle and procoracoid may become lost entirely. An anterior extension of the procoracoid cartilage in the midline frequently splits off to form a distinct element which may become ossified. This is the so-called "omosternum," which in some families, espe- cially in the Polypedatidae and African Ranidae, may become widely forked posteriorly. The sternum is a cartilaginous plate in primitive frogs, and since a sternum is never found among the fossilized remains of branchiosaurs and labyrin- thodonts, it may ha
. The biology of the amphibia. Amphibians. THE SKELETON 237 pitidae, both clavicle and procoracoid may become lost entirely. An anterior extension of the procoracoid cartilage in the midline frequently splits off to form a distinct element which may become ossified. This is the so-called "omosternum," which in some families, espe- cially in the Polypedatidae and African Ranidae, may become widely forked posteriorly. The sternum is a cartilaginous plate in primitive frogs, and since a sternum is never found among the fossilized remains of branchiosaurs and labyrin- thodonts, it may have been represented by a cartilaginous piece in these ancestral groups as well. In primitive frogs the sternum resembles the ab- dominal ribs but possesses anteriorly two leaves fitting between the coracoid carti- lages. In the higher Salientia these leaves are lost and the diverging processes of the sternum fuse to form a single plate which in the many ad- vanced types may become ossified. The sternum of the discoglossids resembles that of urodeles, while that of many higher Salientia is specialized not only by calcifying or ossi- fying but also by assuming a plate or rodlike form. The narrow, bony sternum of Rana represents the extreme condi- tion of this modification. Pelvic Girdle.—The pelvis of the primitive Embolomeri was a distinct advance over the condition found in any fish. It was. Fig. 87.—The pectoral girdles of three neotropical frogs showing the change from the arciferal to the firmisternal type. A. Eleutherodactylus bransfordii. B. Smin- thillus limbatus. C. Rhinoderma Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Noble, Gladwyn Kingsley, 1894-1940. New York : McGraw-Hill
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