IV. VERTEBRATA: MAMMALIA 547 



mal number of young at a birth. Although the mamma? are present in 

 Ijoth sexes, they are functional only in the female, and here only after the 

 birth of the young. 



A dermal skeleton occurs in few species (e.g., the firm l^ony plates of 

 the armadillos) ; on the other hand the axial skeleton shows many features 

 not occurring elsewhere. In the skull many of the Ijones already referred 

 to are evident only as centres of ossification, fusing early with their neigh- 

 bors to form larger bones. As the temporal bone shows, parts of diverse 

 origin may fuse — parts of the visceral skeleton and parts of the cranium; 

 membrane and cartilage bones — so that a sharp line between cranial and 



Fig. 597. — A, true; B, false nipple (after Gegenbaur). 



facial portions cannot be drawn. So it becomes necessary in describing 

 the skull not to follow exactly the model adopted so far, but to take that 

 of human anatomy. 



In the hinder region of the skull is a large occipital bone (figs. 517- 

 519), jointed to the atlas by double occipital condyles, and arising by the 

 fusion of the four occipitalia, p. 458. Besides it includes usually a mem- 

 brane bone, the interparietal, which occurs only in mammals. This is, 

 strictly speaking, a paired bone, arising in the angle between the parietals 

 and the supraoccipital and fusing with the latter. In front of it lie, in the 

 roof of the cranium, as in other vertebrates, the parietals (fused with the 

 interparietals in many ruminants and rodents), the frontals and nasals, 

 the lacrimals being always associated with them. In the floor of the 

 cranium the sphenoid bone hes in front of the basioccipital portion of the 

 occipital. In many mammals this consists of an anterior and a posterior 

 portion throughout life; in man this condition occurs at least in the em- 

 bryo. Each of these parts in development consists of three elements, the 

 posterior with the basisphenoid as the body, and the paired alisphenoids 

 (greater wings) ; the anterior is similarly composed of the presphenoid 

 and the paired orbitosphenoids (lesser wings) (fig. 518, Spb, Ps, Als, 

 Ors) . In front of the sphenoid lies the ethmoid, Eth, likewise formed from 

 three parts, the mesethmoid, which forms a partition between the two 



