SKELETON OF MONOTREMATA. 323 



Four linear impressions upon the upper surface of the skull 

 diverge from the middle of the lambdoidal ridge, and terminate 

 at the temporal ridges. 



The interior of the skull oflPers many unusual modifications. 

 The sella turcica is elongated and narrow in both Monotremes ; 

 it is bounded by two very distinct lateral walls in the Echidna. 

 The posterior clinoid processes are chiefly remarkable for their 

 height in the Ornithorhynchus. The semicircular canals stand 

 out in high relief in this species, as in Birds. In the Echidna 

 the olfactory capsule encroaches upon the anterior part of the 

 cranial cavity in the form of a large convex protuberance, and a 

 very extensive cribriform plate is developed. In the Ornitho- 

 rhynchus the olfactory tract is comparatively small in the form of 

 a depression, and the nerve escapes by a single foramen in the 

 prefrontal : this is likewise an interesting mark of affinity to the 

 Bird and Reptile. But the most remarkable feature in the interior 

 of the skull of the Ornithorhynchus is the bony falx, fig. 204, b. 

 This is not present in the Echidna. The tentorium is mem- 

 branous in both Monotremes. 



C. Bones of the Limbs. — The scapulcB, fig. 199, G and 51, are 

 compressed curved plates, vertical in position, like the other 

 pleurapophyses : they have coalesced with their hajmapophyses, 

 the coracoids, 52 and G, o, which articulate below to the expanded 

 ha3mal spine, called ^ episternum,' t, and also with the succeeding 

 spine, called ' manubrium,' s, or first bone of the true sternum. 

 A dismemberment of the coracoid, n, extends its attachments also 

 to the elongated T-shaped episternum. 



The whole scapula is broader, thicker, and less curved in the 

 Echidna, fig. 201, 5i, than in the Ornithorhynchus. In both 

 Monotremes, the posterior margin or costa is concave, most so in 

 the Ornithorhynchus, and in both it is turned toward the trunk, 

 so that the subscapular surface looks obliquely forward and 

 inward. The articular surface is divided into two facets : the 

 one, internal and flat, articulates with the coracoid ; the other, 

 external, is slightly concave, and contributes, with a similar but 

 narrower concave surface of the coracoid, to form the glenoid 

 cavity for the humerus. 



The coracoid, fig. 199, G, o, and 52, early coalesces with the 

 scapula in the Ornithorhynchus ; it maintains its independent 

 condition to a later period in the Echidna. In both it is a strong, 

 subcompressed, subelongate bone, expanded at both ends : one of 

 these is articulated and anchylosed with the scapula, as above 

 described ; the other is joined to the anterior and external facet 



Y 2 



