COUES ON GEOMYS AND THOMOMYS OSTEOLOGY. 273 
the parietals externally. These are of indeterminate shape, but tend to be 
narrowly rectangular ; and, in Thomomys, a pair of pretty regular linear 
parietals is usually evident. There is constantly an interparietal squarish 
or pentagonal in Thomomysi rather triangular in Geomys, 
With such state of the parietals, there is a corresponding overdevelop 
ment of the temporal bone, especially of its squamosal element, though not 
to the extraordinary extent witnessed in Saccomyidce, where the whole bone 
is blown up like a bladder. The squamosal roofs over most of the cranial 
cavity, and alone forms (with the exception of a little place occupied by the 
interparietal) the whole occipital or lambdoidal crest. The mastoid, which 
persists distinct from both squamosal and occipital, though usually fusing 
with the petrosal, is immensely developed, its superficies lying mostly in, and 
representing about half of each side of, the occipital surface. It develops a 
moderate "mastoid process'', lying against the postero-external corner of the 
squamosal, and looking like a duplicate of the paroccipital process that lies 
against its opposite extremity. The petrosal does not share this unusual 
development, the bullae ossese being, in fact, smaller than they are in Arvicola, 
for instance; they swell but little below the baso-occipital plane. The 
tympanic develops into a tubular meatus, set quite free from its surroundings 
in a deep recess of the squamosal. The petrosal likewise is fissured away 
from the squamosal, but, in adult life, the tympanic, petrosal, and mastoid are 
consolidated. 
The upper and lower parts of the occipital bone are at right angles with 
each other; the basi-occipital is horizontal upon the floor of the skull, while 
the superior and lateral elements are perpendicular behind. The supra- 
occipital is squarish, with rounded corners ; the ex-occipitals develop into 
moderate obtuse processes. Nearly all of the foramen magnum is vertical ; 
the condyles are rather small, and widely divergent superiorly. 
The suture with the basi-occipital, which persists for some time, is ordi 
narily the most conspicuous of the sphenoidal relations which may be appre 
ciated in examination of adult skulls. Close inspection, however, shows the 
squamo-sphenoid suture just inside the glenoid fossa; the alisphenoid barely 
misses taking a part in the mandibular articulation (as in some marsupials) ; 
35 COL 
