SKELETON AND TEETH 63 



neck. The paroccipital processes are bony styles of varying 

 length, which are given off, one on each side external to the 

 condyles. The boundary of the occiput is marked by a ridge, 

 the occipital crest, which varies greatly in prominence, but is 

 very well marked in the morie primitive forms and tends to 

 disappear in the more highly specialized ones. The roof 

 and much of the sides of the cranium are formed by two pairs 

 of large bones, the parietals behind and the frontals in advance ; 

 along the median Une of the cranial roof, where the two parietals 

 meet, is usually another ridge, the sagittal crest, which joins 

 the occipital crest behind. The sagittal crest also varies 

 greatly in prominence, being in some mammals very high and 

 in others entirely absent, and, like the occipital crest, is a prim- 

 itive character; as a rule, it is longest and highest in those 

 mammals which have the smallest brain-capacity. As pointed 

 out by Professor Leche, the development of the sagittal crest 

 is conditioned by the relative proportions of the brain-case and 

 the jaws. Powerful jaws and a small brain-case necessitate 

 the presence of the crest, in order to provide sufficient sm-face 

 of attachment for the temporal muscles, which are important 

 in mastication, while with large brain-case and weak jaws the 

 crest is superfluous. Though the brain-case proper may be 

 quite small, yet it may have its surface enormously increased 

 by great thickening of the cranial bones, as is true of elephants 

 and rhinoceroses, and in them sufficient surface for attachment 

 is afforded to the muscles without the development of a crest. 

 The structure of these cranial bones, more particularly of 

 the parietals, is subject to important changes ; in most mam- 

 mals they are of moderate thickness and have dense layers, 

 or "tables," forming the outer and inner surfaces and, between 

 these, a layer of spongy bone. In many large mammals, 

 however, especially those which have heavy horns or tusks, the 

 cranial bones become enormously thick and the spongy layer 

 is converted into a series of communicating chambers, or 

 sinuses, the partitions between which serve as braces, thus 



