HAPLODONTID Ai—SKULL OF HAPLODON RUFUS. 565 
inelude many details, demonstrating the reaily close relationships of the ani- 
mal to the Sccuride. ‘The interorbital foramen (usually a considerable index 
of affinity in Rodentia), however, is little like that of Sciuromorphs generally, 
wiile it is not at all like that typical of Myomorphs. 
Viewed from above, without taking the rostrum into consideration, the 
general contour of the skull represents a nearly equilateral triangle with 
rounded-off postero-lateral angles, somewhat convex sides, and sinuous base. 
There is a very deep notch or emargination just behind the zygomata, some- 
times almost converted into a foramen by apposition of the posterior angle of 
the zygoma against the wall of the produced auditory bulla. Within this 
triangular contour are a pair of triangular vacuities—the conjoined orbital and 
temporal fossz. The posterior extremities of the zygomata are much farther 
apart than a distance equal to their extreme length. The rostral portion of 
the skull forms rather more than a fourth of the total length; the rostrum is 
stout, its width being contained only about 44 times in the total length; it is 
approximately cylindrical, with nearly parallel sides converging anteriorly, 
and tumid nasal portion. The nasal bones are truncate anteriorly, and do not 
project beyond the plane of the incisive alveoli. From the base of the ros- 
trum on each side, the zygomata suddenly stand off, overshadowing the ante- 
orbital foramina, which are therefore not visible in this view. Thence the 
zygomata curve gently outward and backward, then rather suddenly contract, 
the point at which they are farthest apart being but little in advance of their 
posterior ends. Behind their ends is the deep emargination, as seen also in 
Arctomys, Castor, and other Sciuromorphs, but in this case narrower for its 
depth, appearing almost like a fissure. his separates the zygomatic arch 
from the prominent angular postero-lateral corner of the skull, formed by the 
mastvid and auditory bullz. The posterior outline, on the whole exactly 
transverse, is sinuous, like the human lip or “Cupid’s bow”; for the promi- 
nent sharp occipital ridge which defines the occipital plane from that on top 
the skull, curves backward at first from the corner of the skull and then more 
strongly forward to the median line, this latter anterior convexity sufficing to 
throw some of the occipital surface into view from above. The roof of the 
brain-case is strongly convex and sloping on the sides, though little arched 
longitudinally ; it then spreads more horizontally to form the occipital flanges. 
This surface is marked lengthwise with a pair of raised lines, indicating the 
boundaries of the temporal fossze; at the back border of the orbit, just within 
