PELVIC ARCH • 109 



rise to fenestree closed over by fibrous membrane. In Crocodiles 

 and Chameleons the clavicles are either wanting or rudimentary. 



The presence of a pectoral arch in numerous footless Reptiles (certain 

 Skinks, Amphisbsenians) indicates that they formerly possessed extremities ; 

 rudiments of the latter may even be seen in the embryo though they 

 disappear entirely later on (Angnin fraijilvi). (For the peculiar pectoral 

 arch of the Stegocephala, see Pig. 46.) 



Birds. — In Birds, the scapula consists of a thin and narrow 

 plate of bone often extending far backwards, the strong coracoid 

 being bent at a sharp angle with it in aJl Carinate Birds 

 {Fig. 41). The lower end of the latter is firmly articulated 

 in a groove on the anterior edge of the sternum. 



In almost all Flying Birds the clavicle is well developed, and 

 becomes united with its fellow to form 2.furcula (comp. p. 63 and 

 Fig. 41). It is formed as a membrane bone investing a band of 

 cartilage present in the embryo in this region. 



Amongst the Cursorial Birds, the Emeu and Cassowary possess 

 rudimentary clavicles : in the others they are wanting. They have 

 also undergone reduction in some Carinate Birds {e..g., certain 

 Parrots). 



Mammals. — In Monotremes only amongst Mammals does the 

 coracoid extend ventrally to reach the sternum (Fig. 48) ; in all 

 other members of this Class it characteristically becomes reduced, 

 and simply forms a prominent process on the scapula {coracoid 

 process), which becomes ossified from a separate centre.^ 



Thus the scapula alone serves to support the extremity ; it 

 becomes at the same time greatly broadened out, and gives rise on 

 its outer side — in connection with the highly differentiated muscles 

 of the limb — to a strong ridge (spina scapulas), which extends 

 downwards to form the so-called acromion. The distal end of the 

 clavicle usually becomes connected with the acromion, its proximal 

 end articulating with the anterior edge of the sternum. 



In those Mammals in which the fore-limbs are capable of very 

 varied and free movements, the clavicles are strongly developed. 

 In others, such as the Carnivora and Ungulata, they may be en- 

 tirely wanting or only rudimentary, and in the latter case their 

 relations to the scapula become altered. 



Pelvic Arch. 



Fishes. — The first rudimentary indications of a pelvis are seen 

 in Cartilaginous Ganoids, amongst which, however, they present 

 considerable variations — even in individuals of the same species. 

 They consist of two calcified or even ossified /jclvir plates, which 



' According to Howes the coracoid process represents an epicoraooid (comp. 

 Fig. 48), the coracoid itself being only occasionally indicated by a small centre of 

 ossification on the glenoid margin of the scapula. 



