ir. VERTEBHATA : MAMMALIA. 619 



and beaver (fig. 652, a). More iniportant than these are the modi- 

 fications of dermal glands into mammary or milk glands, which, 

 indeed are so characteristic that they have given rise to the name 

 mammalia. These are almost invariably sebaceous glands (in the 

 monotremes sweat glands) which empty in great numbers upon a 

 restricted area of the skin, which, except in monotremes, is elevated 

 into true nipple (fig. 046, A), or around which the adjacent skin 

 A r> 



Fig. 646.— ^1, true, U, false nipple. (After Gegenbaur.) 



becomes elevated in tubular form {B) as in the cows. The mam- 

 mfB are always symmetrically arranged upon the ventral surface, 

 sometimes in the breast region, but more frequently in the inguinal 

 region. There are at least two, usually more (22 in Centetes). In 

 general the number corresponds to the maximal number of young 

 at a birth. 



A dermal skeleton occurs in few species {e.g.^ the firm bony 

 plates of the armadillos); on the other hand the axial skeleton 

 shows many features not occurring elsewhere. In the skull many 

 ol tlie bones already referred to are evident only as centres of ossi- 

 fication, fu>sing early with their iieighbors to form larger bones. 

 As the temporal bone shows, i)arts 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 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. 

 501, 562), jointed to the atlas by double occipital condyles, and 

 arising by the fusion of the four bones of the occipital region. 

 Besides it includes usually a membrane bone, the interparietal, 

 which occurs only in mammals. This is, strictly speaking, a 

 paired bone, arising in the angle between the parietal and the 

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

 roof ol the cranium, as in other vertebrates, the parietals (fused 

 with the interparietals in many ruminants and rodents), the 

 fiontals and nasals, the lachrymals being always associated with 



