SKULL — MAMMALS 



193 



opisthotic of lower vertebrates. It is vesicular in a few mammals, 

 its cavities being continuous with that of the tympanum. 



Membrane Bones. — The interparietal (large in whales, many 

 rodents and some other groups) is known, besides in mammals, only 

 in some Therapsids. It arises from several centres in the membrane 

 between supraoccipital and the parietals, and usually fuses with 

 either the supraoccipital (Perissodactyls, many Carnivores and 

 Primates) or with the parietals (Ruminants, Sirenia, many rodents) 

 or it may persist as one or two independent bones. When parts 



Fig. 203. — Cranium of Solenodon paradoxus (Gregory, '10). as, alisphenoid; ho, 

 basioccipital; bs, basisphenoid; c, condyle; cf, condylar foramen; e, supraethmoid 

 foramen; eo, exoccipital; /, frontal; fi, incisive foramen; g, glenoid surface; ip, inter- 

 parietal; I, lacrimal; la, foramen lacerum anterius; Im, for. lac. medius; m, maxilla; 

 ms, mastoid; n, nasal; os, orbitosphenoid; p, parietal; pe, petrosal; pi, palatine; p7n, 

 premaxilla; ps, presphenoid; pt, Ppterygoid; so, supraoccipital; sq, squamosal; /, tym-' 

 panic; v, vomer; z, zygomatic. 



remain distinct they are the 'Inca bones,' common in the skulls of 

 the former Peruvians. There are frequently small (sutural or 

 Wormian) bones, of no morphological significance, in the lambdoid 

 suture. 



The parietals are always large, the pair separated by the sagittal 

 suture or this may be obhterated in the adult (primitive Insectivores, 

 bats, some Ungulates, etc.). An exception to this general rule occurs 

 in whales (fig. 225) where the frontals extend to the supraoccipital, 

 forcing the parietals laterally. Ornithorhynchus has a small parietal 



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