a siete 
50 The Extinct Rodentia of North America. [January, 
terminations are somewhat enlarged, In the lower jaw the trans- 
verse ridges are not visible, and there is a low tubercule at’each 
angle of the crown, between which there may be others on the 
border of the crown. Attrition gives the grinding surface of the 
latter a basin-like character. The foramen wfraorbitale is a short, 
narrow fissure, situated in the inferior part of the maxillary bone 
in front of its tooth-bearing portion, but descending to the level 
of the alveolar border. 
The well-known characters of this genus are found in the man- 
dibles of species which I ob- 
tained from the White River — 
Miocene beds of Colorado 
and the John Day of Oregon. — 
The teeth display the sub- 
quadrate form of this genus, 
without any tendency to the © 
Spermophilus. Two of the — 
species, S. vortmani Cope and ~ 
S. relictu$ Cope are as large as | 
z our gray and red squirrels, | 
Fic, 4.—Skuil and jaws of species of Sci- TTF ectively , and the third, 5. ; 
urus; original, from Vol. rv Report U. S. Ge- bal/ovianus Cope; is about the : 
er eiea igs aM, S. ballovionus: size of the Tamias quadrivi- 
larged. Figs. e-f, S. relictus, enlarged one- fatus or Western chip-munk. i 
half. g-h, S. vortmani, natural size. ke S rie Gs kon ia 4 
White River formation, and the two other species from the John | 
Day. . i 
GYMNOPTYCHUS Cope. : 
In dentition this genus is much like Ischyromys. There arè 
only four superior molars, 
As compared with the existing genera of squirrels, it differs in — 
the structure of the molar teeth. The arrangement of the tuber- — 
cles and crests is more complex than in any of them, excepting — 
Pteromys. Thus in all of them there is but one internal crescent — 
of the superior molars, and but two or three cross-crests; while in — 
the inferior molars the arrangement is unlike that of the superior 
teeth, the cross-crests being marginal only. In Pteromys (F. . 
- Cav.) the transverse valleys of the inferior series of Gymnopty- — 
