SKULL MAMMALS 



211 



distinct. The maxillahas a large infraorbital foramen and the orbital plate of 

 the palatine may be membranous, leaving a gap in the orbit of the dry skull. 



Creodonta (fig. 223) are apparently the parent stock of the Carnivores, 

 and also have Marsupial affinities. They always have a separate optic foramen; 

 usually an aUsphenoidal canal occurs, there is a large facial part to the lacrimal, 

 the hard palate is complete and the angle of the lower jaw is not inflected. 



Fig. 223. — Skull of Mesonyx (Scott, '86) 



Cetacea (Cete). — The three suborders of whales differ considerably in 

 cranial structure. All have spongy bones and the part of the cranium containing 

 the brain never exceeds a quarter of the cranial length. Archicetes and Mysta- 

 cocetes (fig. 224) have a symmetrical cranium, but Odontocetes are more or less 

 asymmetrical, the left premaxilla and maxilla being larger than the right, these 

 features reaching their extreme in Physctcr and the narwhal. In living species 

 the supraoccipital (with the interparietal) meets the frontals, excluding the 



Fig. 224. — Skull of Balcenajaponica (Boas, in Schimkewitsch) . c, occipital condyle; 

 dO, exoccipital; /, frontal; /, lacrimal; mx, maxilla; n, nasal; p, parietal; fa, palatine; pm, 

 premaxilla; pi, pterygoid; so, supraoccipital; sq, squamosal; i, tympanic; z, zygomatic. 



parietals from the middle line, the separation of these being carried so far in 

 Odontocetes (fig. 225) that these bones form no part of the cranial roof, a condi- 

 tion without parallel in other mammals. The relations are more normal in 

 Archicetes. Zeuglodon and Mystacocetes have the nares at about the middle of 

 the snout and roofed by long and slender nasals; in Odontocetes the united 

 nares are at the base of the snout, the narial passages are nearly vertical and the 



