MONOTREMATA. 



239 



the long head of the triceps muscle takes origin. This 

 ridge then answers to the posterior or glenoid border of the 

 ordinary Mammal, and the hinder border of the Monotreme's 

 scapula {cb) would correspond to the anterior or coracoid 

 border. If this is really the case, the scapula of the Mono- 

 treme and that of the Cetacean offer the widest contrast, 

 the supposed primitive trihedral rod being flattened in 

 opposite directions. In the Cetacean scapula there are two 

 nearly parallel surfaces, the postscapular and the subsca- 

 pular fossae ; while the third, the prescapular fossa, is reduced 

 to the smallest possible width — quite obsolete, in fact — in 

 Platanista. In the Monotreme the last-named fossa is so 

 expanded that the other two, instead of being parallel to 

 each other on opposite sides of the bone, are brought almost 

 into one plane, which is parallel and opposite to the sub- 

 scapular fossa. 



The coracoid {c) is a stout subcylindrical bone, expanded 

 at its extremities, taking at its upper end a considerable 

 share in the formation of the glenoid cavity, and becoming 

 firmly ankylosed with the scapula. At its lower end it 

 articulates to the side of the presternum, just in front of the 

 first rib. 



Placed in front of the inner end of the coracoid is a 

 broad, flat, shield-like plate of bone {eptcoracoid, ec\ the 

 rounded inner border of which passes beyond the median 

 line, overlapping the corresponding bone of the opposite 

 side. In the Echidna the left lies superficially to the 

 right, while in the Ornithorhynchus this disposition is 

 reversed. 



Upon the front end of the presternum, lying below its 

 anterior continuation (p7-oosteo/i, see p. 84) and also below 

 the epicoracoids, is a large azygous I'-shaped bone {ic, see 

 also Fig. 43, p. 85), which has no homologue in any other 



