COUES ON GEOMYS AND THOMOMYS OSTEOLOGY. 
277 
Pterygoids ? (will be found differ 
ing appreciably from those of Tho- 
momys). 
Pterygoids appearing like a bifurca 
tion into two thin diverging plates 
of a single median vertical palatal 
plate. 
In like manner, we may proceed to compare some of the principal cranial 
characters of Geomyida and Saecomyida. Notwithstanding the unquestion 
ably close affinity of these two families, which must stand next to each other 
in the system, their crania are curiously different in general appearance and 
details of contour. The discrepancies are, however, of a superficial charac 
ter, resulting mainly from the extraordinary molding of the parts in Sacco- 
myid<. In other words, it is a matter of mere shape, for the most part. 
There are, however, some curious and more essential features, of which the 
enormous inflation of various elements of the temporal bone and peculiar 
zygomatic relations posteriorly are the most remarkable. Probably, going 
into details, a hundred actual differences between the skulls of Geomyida and 
SaccojJiyida might be enumerated. I shall content myself with tabulating a 
few 'of the more important of these. The comparisons are made between 
Geomys'bursarius and Dipodomys ordi; it should be remembered, however, 
that the latter presents an extreme case, the average characters of the Sacco- 
myid(B being less different from those of Geomyida. 
GEOMYID.E. 
Skull massive, angular, in general like 
that of Arvicola, &c. 
Interorbital space the narrowest part 
of the skull narrower than ros 
trum. 
Occipital region approaching a plane 
surface, without median emargina- 
tion. 
Nasal bones not produced beyond 
SACCOMY1D.E 
Skull thin and papery, the corners 
rounded off; the resulting general 
shape peculiar. 
Interorbital space expanded, very 
much broader than the rostrum. 
Occipital region formed chiefly of 
enormous bulging mastoids, with 
deep median emargination. 
Nasal bones produced far beyond in- 
