8 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
large ones at bases of Ist, 2d, and 5th toes, respectively ; and one at con- 
joined bases of 3d and 4th toes; those at the bases of all the digits, except 
the hallux, being more or less confluent.* Pelage soft, lustrous, white 
below. 
To the foregoing characters, rather descriptive than simply definitive, 
_ and indicating little else than an overgrown Hesperomys, we may add the fol- 
lowing more diagnostic features, derived from the skull and teeth: 
Skull elongate, twice (at least) as long as wide, in spite of the divergent 
zygomata; these do not sink to the level of the palate, and turn toward the 
squamosal almost at an angle (¢f Hesperomys). Maxillar boundary of ante- 
orbital foramen developing no pointed process (¢f Sigmodon). Palate ending 
as a simple emarginate or concave shelf, opposite the interspace between last 
and penultimate molars; the palato-maxillary suture opposite the interspace 
between first and middle molars (cf any other sigmodont genus). Incisive 
foramina very short. Foramen magnum broader than high. Auditory bulla 
rather small; their axes very oblique to the axis of the skull. Nasal bones 
not reaching as far back as nasal branches of intermaxillaries, which gain the 
interorbital region. No definite bead on upper margin of orbits. Inter- 
parietal bone subquadrate, but with a large, well-defined spur on each side. 
Posterior aspect of skull truncate ; 7. e., the occipital plane is about perpendic- 
ular, meeting the flattened superior surface of the skull nearly at right angles 
(in all other genera, the coronal rounds more or less gently into the occipital 
surface). Under jaw with long, acute, coronoid process, overtopping condyle ; 
root of the under incisor causing a moderate protuberance on the outside 
of the jaw, rather at the root of the condyle itself than at the notch between 
condyle and coronoid. Teeth of the ordinary sigmodont pattern; nearest to 
Hesperomys proper, and, as in that genus, decreasing regularly in size from first 
to last—in the upper jaw at any rate; in the lower, the last tooth conspicuously 
smaller than either of the other two. All the upper teeth with usually two 
external and one internal reéntrant loops of enamel (but the first often with 
a supplementary internal loop, and the last often with only one external loop). 
First and second under molars each with two internal and two external reén- 
trant loops; last under molar with one of each. In unworn teeth, all the 
reéntrances open, the saliencies correspondingly sharp and divaricating, thus 
simulating the prismatic structure of Arvicoline ; in old teeth, however, these 
“The tubercles at bases of all the exterior fingers and toes show a tendency to develop little 
accessory tubercles upon their outer faces. 
