174 HISTORY OF THE HUMAN BODY 



cartilage. In teleosts the cleithra become excessively developed 

 and function as the entire girdle, while the clavicles are want- 

 ing and the cartilaginous element is much reduced. There are 

 also other small dorsal pieces, the supra-cleithra (post-tem- 

 poral and supra-clavicle of many authors), which connect the 

 girdle with the skull. 



As the teleosts represent the perfection of the fish type, but 

 are not in the direct line of descent, their condition represents 

 a specialization not closely related to higher forms, and the 

 direct history is carried over, with a small interval, from the 

 ganoids to the amphibians. In these latter the dermal element 

 is but little in evidence, while the cartilaginous part is volu- 

 minous and shows centers of ossification. In urodeles each 

 half-girdle is in the form of a thin plate, wrapped about the 

 side of the body and incompletely divided into three portions, 

 a dorsally extended scapula, containing an ossified area, and 

 two ventral extensions separated by a notch, the coracoid and 

 procoracoid, both cartilaginous. There are here no dermal 

 elements. In the tailless amphibians the cartilaginous founda- 

 tion is much the same as in urodeles, but there is also an ossi- 

 fied area in the coracoid, and a dermal clavicle, in the form 

 of an inverted trough, which fits closely over the procoracoid, 

 forming a compound piece. The two lateral halves become 

 also more or less closely associated with median sternal (and 

 episternal) elements, and in the more specialized frogs the 

 whole forms a complicated skeletal armatuie protecting the 

 vital organs and forming a functional thorax, something like 

 that of higher forms, although without rib components and 

 not involved in the respiratory process. 



In the Amniota there appear three parts to the girdle, scap- 

 ula, coracoid, and clavicle, corresponding in the main to those 

 of Amphibia. The two first are preformed in cartilage and 

 ossify later on in development ; but the clavicle of reptiles and 

 birds has no cartilaginous stage, and seems thus to represent 

 the dermal element alone. In birds the two clavicles fuse with 

 a median interclavicle (possibly an episternum), to form the 

 furcula or " wish-bone." In mammals the scapula receives 

 the most emphasis, while the coracoid is never present as a 



