1002 
Journal of Agricultural Research 
Vol. XXVIII, No. 10 
animals that are barely matured, and upon which there is not as yet pronounced 
temporal ridging, and the variation is never so decided and abrupt as in the 
latter character. In juveniles the lambdoidal crests are inconspicuous. 
There is some slight variation of the exoccipitals, and in young animals the 
sutures between these and the mastoids are relatively unraised. There is much 
variation, however, in the development of the paroccipital processes. In all 
but the smallest juveniles there is little age variation, but in these, although the 
processes are of respectable length, they closely embrace the bullae and are con¬ 
sequently directed slightly forward of the perpendicular. In adults and sub¬ 
adults they are, as a rule, only slightly longer proportionately, but the distal 
extremities are usually free from the bullae and are directed slightly rearward 
of the perpendicular. The variation is chiefly in the length and the degree of 
the infero-medial curvature of the tips, which in most of the specimens is moder¬ 
ate. The sole function of these processes seems to be for the attachment of the 
digastric muscles. This being the case, it appears that these processes are some¬ 
what better developed in juveniles than the condition of the general muscula¬ 
ture of the head at this age might lead us to expect, pointing to the probability 
that the digastric may be of more than ordinary importance in the economy of 
these mammals. In juveniles the margins of the basioccipital closely embrace 
the audital bullae, but in older examples there are vacuities at the lateral mar¬ 
gins, these depending upon variation in the width of a slight constriction of the 
bone directly posterior to the basioccipital-sphenoid suture. 
Interparietal. —This bone articulates anteriorly with the parietals, laterally 
to a varying extent with the squamosals, and posteriorly with the supraoccipital. 
It is highly variable in shape not because of any muscular attachments, but for 
the reason that its position renders it particularly subject to distortion by the 
development of adjoining parts of the cranium. The antero-posterior dimension 
varies from about 3.1 to 3.7 mm., and the transverse one from 6.1 in a very old 
individual to 8 mm. In the majority of individuals this bone is almost exactly 
twice as long as wide. There seems to be very little variation from juvenility 
to middle life. In the largest adults, however, the relative, and even actual, 
decrease in the prelambdoidal width of the braincase, with consequent narrow¬ 
ing of the distance between the temporal ridges, makes this bone appear narrower. 
In addition, the fact that it is narrower in some of these examples can not be 
ignored. This state of affairs may be attributable to strains imposed upon it by 
a greater growth force in the development and encroachment of the squamosals, 
with consequent absorption of the lateral margins of the interparietal, an 
explanation that is entirely logical if this really be the weaker bone, as seems to 
be the case. 
Variation in the precise form of the anterior outline of the interparietal is 
almost infinite. In a few specimens this follows the form of a gentle, convex 
arc. In many others an almost straight line is broken by a rather sharp projec¬ 
tion of the medial portion into a point. In the majority the sutures with the 
parietals follow a very wide obtuse angle, while in a number of others this angle 
is somewhat sharper and less obtuse. There is such variation, however, that it 
is not practicable to divide the series into definite groups based upon this character. 
The suture with the squamosal may form almost a right angle with that with 
the occipital, but in the majority of cases the angle is of about 30° to 35° (the 
apex directed laterally). In still others the angle is considerably sharper, due to 
a greater prolongation of its apex. (See lower detail, fig. 18.) In almost all 
cases this suture pursues a very erratic course. That with the supraoccipital 
is usually practically straight or very gently curved. A slight, very obtuse 
