268 EXPLORATION OF THE CANONS OF THE COLORADO. 
though specimens differ in this respect, owing to a variable curve of these 
parts. In adult Geomys, the case is as stated, the zygomata converging a 
little backward in a nearly straight line, so that posteriorly their width apart 
is little if any greater than the intermastoid diameter of the skull. In Tho- 
momys, there is a more decided outward convexity of these arches, and their 
greatest width apart is nearly at their middle if anything, posterior to this, 
and at any rate the width here decidedly surpasses the intermastoid diameter. 
In front, the zygomatic plates of the maxillaries start out at nearly a right 
angle with the long axis of the skull ; behind, the zygomata curve rather 
abruptly into the squamosal. There is a deep abrupt emargination behind 
the posterior root of the zygomata, between this and the postero-lateral corner 
of the skull ; in the recess, the tubular bony meatus auditorius appears pro 
truding in this view. The lambdoidal crest, forming the posterior boundary 
of the skull, is a slight curve, more or less irregular ; most of it is squamo 
sal, for the occipital bone rises to this crest for only a short distance. The 
narrowest part of the skull is between the orbits, where the width is less 
than the diameter of the rostrum. The irregularly pyriform figures, circum 
scribed by the zygomata and walls of the cranium, are of large size ; no 
orbits proper are defined in the general orbital space, owing to deficiency of 
both pre- and post-orbital processes. The dome of the cerebral cavity is but 
little inflated ; its sides seem somewhat pinched, there being a decided though 
shallow concavity just above the zygomatic spur of the squamosal ; and a 
slight bulging anteriorly on each side at the usual site of post-orbital pro 
cesses. The median line of the cerebral roof, in an old Geomys skull, is a 
ridge ; this ridge bifurcates anteriorly to send a curved leg forward and out 
ward to the orbital margins ; and behind enlarges a little to receive a small 
interparietal. In various . Thomomys skulls of different ages, the squarnosals 
leave a rectangular interval occupied by small, narrowly linear parietals ; and, 
instead of a single median ridge, there are two parallel ridges, with a depressed 
interval. The sides of the rostrum are straight and parallel, the edge being 
the swollen track of the superior incisors. The end is vertically truncate; 
the tips of the nasals and interma'xillaries and the faces of the incisors being 
all about in one perpendicular plane. The width of the rostrum is rather 
more than half its length. 
