444 University of California Publications in Zoology — (Vou. 17 
the zygomatie arch is often different in animals of apparently the 
same age. The arches are bowed outward more in some specimens 
than in others. The postzygomatie noteh is variable. The degree 
of development of the lambdoidal crest varies in animals of apparently 
comparable age, being accentuated in certain specimens and setting 
off a considerable fossa anterior to it, but in others being compara- 
tively weakly developed. 
Perhaps the most variable of the cranial characters are furnished 
‘by the bones on the posterior aspect of the skull (see text figs. B, C, D). 
In the adult, the sutures between the exoccipitals, supraoccipital, and 
the basioccipital have completely disappeared. The paroccipital pro- 
cess and the oceipital condyle belong to the exoecipitals. The sub- 
stance of the mastoid process, as viewed on the posterior aspect of the 
cranium, results from the ankylosis of two elements, one, the inner, 
from the exoecipital, and the other, the outer, from the squamosal. 
A plate from the latter bone invades the outline of the mastoid process 
from below, and is marked off by prominent sutures in animals of 
every age. The degree of expansion of the winglike mastoids, the 
degree of development of the paroccipital processes, and particularly 
the degree of invasion of the plate of the squamosal which comes up 
from below, are subject to variation, the wide range of which suggests 
that it may be due to age. Individual differences are observable in 
the outhne of the foramen magnum, which is rounder in some speci- 
mens, flatter in others. In the ventral aspect of the skull one notes 
that there is considerable variation in the diameter of the third upper 
premolar, and noticeable variation in the outline of the hamular pro- 
cesses, the length of the incisive foramina, the outline of the inter- 
pterygoid fossa, the outlne of the zygomatic arch particularly poste- 
riorly, the caliber of the audital tubes and the degree of expansion 
and development of the paroccipital processes. The relations of the 
foramina in the regions of the foramen lacerum medium are subject 
to much variation. 
Looking at the skulls in their anterior aspect it is of interest to 
note the variation in size of the infraorbital foramen, which in its 
ereatest diameter, ranges, in different specimens, from about 2.6 to 
4.0 millimeters. 
The outline of the coronoid process of the mandible varies from 
a form in which it is searcely hooked backward at all, to one in which 
the hook form is prominent. The mass and outline of the econdylar 
process undergo considerable change from one individual extreme to 
