THE SKELETON IN MAMMALIA. THE SKULL. 473 



periotic and tympanic are fused together, but not as a rule 

 to the squamosal. The ascending portion of the mandible is 

 very high and broad, the angle rounded and the coronoid 

 process moderate in size. The hyoid is singular, there is a 

 large flat basi-hyal prolonged laterally into two broad flattened 

 thyro-hyals. Articulating with its anterior end are two large 

 triangular cerato-hyals, which are drawn out into two processes 

 meeting in the middle line. 



AMBLYPODA. In the Uintatheriidae (Dinocerata) the skull 

 has a very remarkable character, being long and narrow and 

 drawn out into three pairs of rounded protuberances, a small 

 pair on the nasals, a larger pair on the maxillae in front of the 

 orbits, and the largest pair on the parietals. The cranial cavity, 

 and especially the cerebral fossa, is extraordinarily small. The 

 orbit is not divided behind from the temporal fossa. The 

 mandible has a prominent angle, and a long curved coronoid 

 process ; its symphysial portion bears a curious flattened out- 

 growth to protect the great upper canines. 



In Coryphodon the skull is of a more normal character, 

 being without the conspicuous protuberances. The cranial 

 cavity though very small is not so small as in Uintatherium. 



PROBOSCIDEA. The character of the skull in the young ele- 

 phant differs much from that in. the old animal. In very young 

 individuals the skull is of a normal character, and the cranial 

 cavity is distinctly large in proportion to the bulk of the skull. 

 But as the animal gets older, while its brain does not grow 

 much, the size of its trunk and especially of its tusks increases 

 greatly ; and consequently the skull wall is required to be of 

 very great superficial extent in order to afford space for the 

 attachment of the muscles necessary for the support of these 

 heavy weights. This increase in superficial extent is brought 

 about without much increase in weight of bone by the develop- 

 ment of an enormous number of air cells in nearly all the 

 bones of the skull ; sometimes, as in the case of the frontal, 

 separating the inner wall of the bone from the outer, by as 



