47G COMPARATIVE ANATOMY. 



independent periosteal ossification. The ossification sometimes ex- 

 tends from the scapula on to the precoracoid. In the Anura the 

 two ventral processes (Fig. 262, A co cd) on either side of the 

 shoulder-girdle are united by their cartilaginous ends, and may also 

 become united in the middle line (liana). In this case there is a 

 foramen on either side, in the ventral portion of the shoulder-girdle. 

 The coracoid (co') is ossified independently, while the precoracoid 

 becomes closely related to the clavicle (d). 



In the Reptilia, likewise, each half of the shoulder-girdle forms a 

 single piece, which closely resembles in form the same part in the 

 Amphibia. The coracoid, which is generally broad, is not unfre- 

 quently fenestrated (Saurii). A process of the scapula, which is 

 merely indicated in the Amphibia, is converted into the acromion, 

 and unites the scapula with the clavicle (Fig. 202, C d). In the 

 Chelonii the scapula is generally a cylindrical bone (B s), which 

 forms an angle with, and is directly continuous with, the precoracoid 

 (B co) at the glenoid cavity. The end of this precoracoid is 

 connected by a ligament with the cartilaginous end of the coracoid. 



In the Crocodilini the precoracoid has completely disappeared, 

 so that the scapula and coracoid alone make up the shoulder-girdle. 

 In Birds there is a somewhat similar arrangement ; the small and 

 slightly curved scapula is united to the strong coracoid at the 

 glenoid cavity ; the coracoid itself is, as in Reptiles, attached to the 

 plate of the sternum. The Ratitaj indicate their closer affinity to 

 the Saurii by the presence of a rudiment of the precoracoid. 



Among Mammals the coracoid is complete in the Monotremata 

 only. In the rest, the only sign of it is the process (coracoid process) 

 which is given off from the scapula, and lies in front of the glenoid 

 cavity ; it is in rare cases only that the sternal end of the coracoid 

 persists. I have discovered it, however, in Sorex and Mus, where it 

 forms a piece of cartilage attached -to either side of the manubrium 

 sterni. The scapular remnant of the coracoid still continues to take 

 part in the formation of the glenoid cavity, but this share decreases 

 as that of the scapula increases, so that at last this latter bone alone 

 forms the support for the anterior extremity, which thereby acquires 

 a greater power of free movement. The primitive independence of 

 the remnant of the coracoid is implied by the presence in it of a special 

 centre of ossification, which persists so long as it is not completely 

 fused with the scapula. 



In form, the scapula of Mammals resembles that of the Reptiles, 

 but owing to the presence in it of new constituents it differs from the 

 latter in some essential points. In the Monotremata there are indi- 

 cations of a spine, the end of which forms the acromion. In the rest 

 of the Mammalia the lateral edge of this broad piece is developed 

 into a larger ridge, which now, owing to the development of 

 the median ridge also, into a projecting plate of bone, or spina 

 scapulas, marks off a superior and an inferior fossa. The 

 anterior end of the spine is always developed into an acromial 

 process. The most important of the other changes which occur in 



