SCIURIDAI—SYNOPSIS OF GENERA. C41 
TH. Skull short, broad, and rounded ; postorbital processes well developed, slender, directed downward 
and backward; zygomatic arches usually slender, the malar expanded in a nearly vertical plane, 
sometimes turned slightly outward dorsally ; anteorbital foramen a narrow slit, anterior to the 
zygomatic process of the maxillary; upper premolars either two or one (two as arule); when 
two are present, the first is always small and sometimes deciduous; ears moderate, well clothed, 
and sometimes conspicuously tufted ; no cheek-pouches; nail of pollex rudimentary; pelage 
generally soft, but occasionally coarse and more or less rigid; tail broad, with the hairs mostly 
Sircotcomatoral yy mererreen crates cetiere an) dts apa neeena ea sea ae cistoicece cone occ ae Sciurus, 
TY. Skull, in general form, intermediate between that of Sciurus and Tamias ; postorbital processes 
smaller, and the nasal bones narrower (at least than in Sciurus); no cheek-pouches; ears very 
small or rudimentary ; tail short, but slightly flattened ; pelage harsh, consisting of flattened, 
grooved spines and coarse bristly hairs, with little under fur...-...-..-..--2...--...-. - AXerus. 
V Skull narrowed anteriorly ; postorbital processes very slender, directed downward and backward ; 
zygomatic arches more expanded and depressed anteriorly, and the plane of expansion of the 
malar more oblique; anteorbital foramen oval, situated in the base of the zygomatic process 
of the maxillary; upper premolars either two or one; when two are present, the first is gener- 
ally minute ; large internal cheek-pouches; ears of medium size, well clothed, but never tufted; 
pollex with a well developed nail; tail rather broad, but shorter and much narrower than 
in Sciurus and Sciuropterus; pelage soft; dorsal surface with longitudinal stripes; size rather 
GHEE Wleocoecenonde ab antess Gadiéos n55d CaS cab Sdaees eeSeoSresEedoS Soe oeBoeese doooehsacsed Tamias, 
VI. Skull variable in form, being either narrow and elongate or short and broad, much as in Sciurus ; 
postorbital processes generally triangular, strong, and directed downward; zygomatic process 
of the maxillary depressed and expanded ; plane of the malar bone turned outward, sometimes 
expanded nearly horizontally; zygomatic arches spreading; anteorbital foramina placed 
more anteriorly than in Yamias, much as in Seiurus, but rather larger and more circular, 
with a strongly developed tubercle at the outer lower border; upper premolars always two 
the first variable in size, but always larger than in the preceding genera, in some species rela- 
tively nearly as large as in Cynomys; grinding-teeth in some species nearly as in Sciurus, in- 
creasing in size in other species till nearly as strong as in Cynomys; cheek-pouches well devel- 
oped; body slender or thick-set ; tail long,moderate, or short; ears large, medium, or rudiment- 
ary, never tufted; nail of pollex generally 1udimentary ; character of the pelage and pattern of 
GOO RDO WATE G5 -555 Sob cesier cbs a5 55 SSC OBES HEC BSSESU ne SeSG COB Seu DobeeeIgcons Spermophilus. 
VII. Skull strong and massive, rather short and broad; zygomatic arches widely expanded; malar 
rather slight, its plane oblique; muzzle narrow; postorbital processes long, strong, and directed 
downward; anteorbital foramina rather large, subtriangular, thrown outward basally, and 
placed well forward ; upper premolars two, the first large, of the same breadth antero-posteri- 
orly as the other grinding-teeth ; dentition very heavy, the grinding-teeth with the transverse 
breadth twice the antero-posterior, the last molar much larger than the others, and subtriangu- 
lar ; the molar series strongly convergent posteriorly ; body heavy, thick-set; cheek-pouches 
not large; tail short; ears rudimentary ; nail of pollex well developed ; pelage short and rather 
Ripe Meee oye se aloes ae ee cence coe a tac col sete s stb ccs Scie cceicels ve nate wauien emulates Cynomys. 
VIII. Skull with the dorsal outline nearly straight; frontal region flat or depressed; in other respects 
the form is much as in Cynomys. but with heavier postorbital processes, not (or but slightly) 
decurved, much weaker dentition, and the molar series parallel; interparietal and occipital 
crests much more strongly developed than in the other genera; size large; body stout, broad, 
depressed ; cheek-pouches absent or small; tail short, bushy; ears small; nail of pollex broad, 
AL eel Ae OI CORTSO sO V Vaca ane! awe boos clesscoe ean cctloc es sa ee acta ns anecisani cocmmn Arctomys. 
In the first five genera, the skull is more or less convex above, being in 
the first three highly arched, but generally much less so in Spermophilus, and 
still less in Cynomys, while in Avctomys it is nearly straight. The postorb- 
ital processes become much heavier in Cynomys than in the preceding genera, 
but reach still greater development in Arctomys. The skull is generally des- 
titute of ridges for muscular attachment, but such ridges are generally trace- 
able in aged individuals, even in Sciwrus, in which, however, they are widely 
separated, converging posteriorly. In most of the Spermophiles, they are more 
41 M 
