498 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
articulation with the rostral bones, and with straight sides convergent posteri- 
orly.* There is a large lachrymal of very irregular shape, extensively scroll- 
like, very delicate in texture, and loosely attached; it closes a large aperture 
leading into the nasal chamber. The orbit is also perforated behind by a 
single very large foramen of exit of cranial nerves. It is bounded in front, 
but not roofed over, by the zygomatic plate of the maxillary, not noticeable 
in character. The extremely delicate malar sutures in front for a long dis- 
tance, clasp-like, against the zygomatic process; behind, it simply abuts 
against a slight heel of the squamosal, almost in relation with the tympanice. 
The singularly displaced ‘‘anteorbital” foramen is a large rounded aperture 
in the side of the snout, communicating directly with the nasal cavity. The 
nasal bones are parallel-edged for most of their length, but widen a little and 
become semitubular anteriorly where they project; they are truncate behind, 
reaching opposite the middle of the jagged fronto-maxillary suture. The 
sides of the rostrum are contracted below, leaving a very narrow bridge of 
bone between molars and incisors; the contracted incisive foramina are 
bounded behind by the maxillaries, though they are chiefly pierced in the 
intermaxillaries. The intermolar portion of the palate is longer than wide, 
and a little convergent anteriorly ; the maxillo-palatine suture is opposite the 
second molar; there is a pair of deep palatal pits opposite the last molars; 
behind there is a pair of much larger vacuities bounded by palatals in front, 
sphenoid behind, and pterygoids internally. The latter are simple, straight, 
nearly parallel processes, bounding the contracted posterior nares, and abut- 
ting against the petrosals. The orbital plate of the sphenoid is of moderate 
extent, owing to the size and site of the squamosal. 
The molars in this genus, as in others of the family excepting Dipodomys, 
are all rooted. They have been said to have four roots, but such is not the 
case in the specimens I have examined. In P. peniciélatus, all the upper 
molars have three roots apiece, and all the under molars have two roots 
apiece, excepting the back upper one, which has but one. The front upper 
molar has one root in advance, corresponding to the anterior lobe of this 
tooth, and a pair of roots obliquely side by side behind. The next two upper. 
molars have each an exterior pair of roots, lengthwise, and a larger single root 
*Neither this nor any other North American genus shows the ridge of the frontal, which in Heteromys, 
as figured by Peters, makes a boss, or bead, along the margin of the orbit,and thence extends into the 
parietal region. This would seem to be a good character of Heteromyina ; and the skull of the latter is 
peculiar in other respects—to judge from Peters’s figure. 
