492 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
or not attaining top of condyle; mandible small and weak in comparison with 
rest of skull. Cervical vertebrae* sometimes anchylosed. Incisors variable 
(the upper compressed and sulcate in North American genera, said to be broad 
and smooth in Heteromys). Molars (2. e. pm. and m.) 4-4, rooted or rootless. 
General form Murine; body slender and gracefui; ears and eyes well devel- 
Hind limbs somewhat or very decidedly saltatorial. 
oped. Tail as long as 
the body (more or less). Fore claws moderate, not obviously fossorial. 
External cheek-pouches ample, not connected with the mouth, furry inside, 
furnished with a special muscle (these pouches as in Geomyida, but no other 
Rodents). 
with flattened spines (in Heteromys); always without under fur. 
Pelage generally coarse and hispid, said to be sometimes mixed 
The species 
very active and quick in their movements, like Mice; some of them leaping 
like Jerboas. 
The foregoing characters amply sufhice for the determination of this 
family, though others might be adduced. The skull is very peculiar; its 
main features result from the exaggerated state of the mastoids, which pro- 
duce great width behind, roof much of the cerebral cavity, pinch the occip- 
ital, and push the squamosal into the orbit. The next most prominent features 
The 
curious position of the anteorbital foramen is shared by the Geomyide. To 
are the slight thread-like malar, attenuated rostrum, and flat palate. 
bring the comparative characters of Saccomyide and Geomyide into stronger 
relief, the following may be adduced :— 
GEOMYID&. SACCOMYID. 
(Geomys and Thomomys.) (Perognathus, Cricetodipus, Dipodomys.) 
Skull massive, angular, in general ofan Arvicoline 
superficies. 
Interorbital constriction narrower than rostrum. 
Interzygomatic width the greatest diameter of 
the skull. 
Palate strongly sloping downward far below level 
of zygomata. 
Nasals not produced beyond incisors; rostrum 
broad, blunt, parallel-sided. 
Zygomata strong, flaring, with stout, short malar, 
having ordinary connections. 
Frontal compressed. 
Parietals compressed, irregularly linear, remote 
from orbits. 
Skull delicate, with rounded-off angles and slight 
ridges, if any. 
Interorbital space much broader than rostrum, 
Intermastoid width the greatest diameter of the 
skull. 
Palate nearly horizontal, little, if any, below the 
level of zygomata. 3 
Nasals produced beyond incisors; rostrum coni- 
pressed, tapering, acute. 
Zygomata slender, parallel, with long thread-like 
malar, almost oractually abutting against tympanic. 
Frontal very broad. 
Parietals broad, triangular or pentangular, com- 
ing to edge of orbits. 
* There is a curious coincidence or correlation between the elongation of the hind legs in adaptation 
to saltatorial progression, and a special condition of the cervical vertebra and of certain elements of the 
temporal bone, such development of the hind legs being often associated with anchylosis of cervical 
vertebra, and great inflation of the bones of the ear. Dipus and Dipodomys illustrate these conditions 
very strongly. 
