SCIURIDZ—EXTINCT SPECIES. 933 
ARCTOMYS VETUS Marsh. 
Arctomys vetus Marsu, Amer. Journ. Sci. and Arts, 3d ser. ii, 1871, 121. 
About one-third as large as Arctomys monax. Described from a near! y 
perfect lower jaw and other remains. Lower incisors with a shallow median 
groove on the anterior surface. Loup Fork, Northern Nebraska; Pliocene. 
Gmnus PARAMYS Leidy. 
Paramys Lewy, Proc. Acad. Nat. Sci. Phila. 1871, 231; Ann. Rep. U. S. Geol. Sury. Terr. for 1871 (1872), 
357; Extinct Vert. Fauna, 1873, 109. 
Although half a dozen or more species of Paramys have been de- 
scribed, the genus is thus far known only fronf more or less imperfect 
mandibular rami. It was first described by Dr. Leidy, in 1871, from remains 
discovered by Dr. Carter in Tertiary deposits near Fort Bridger, Utah, who 
recognized three species from as many fragments of lower jaws. Professor 
Marsh and Professor Cope have each since described a single additional 
species from the Tertiary of Western Wyoming and Colorado. The largest 
of these species (Paramys robustus Marsh) exceeds in size the common 
Marmot (Arctomys monax), while the smallest (Paramys relictus Cope, per- 
haps not strictly referable to this genus) is not larger than the common Chick- 
aree (Sciurus hudsonius). As in all the other genera of the Sccuride, the 
lower grinding teeth are four in number on each side, with short, square, 
tuberculate, enameled crowns, with the same concave surface and tuberculate 
corners as in other genera. 
“The lower jaw is proportionally shorter and deeper than in most known 
Rodents, the reduction in length being mainly due to a less development of 
that part of the bone in advance of the molars. To compensate for differ- 
ence in length, and to make room to accommodate the incisors, these teeth 
reach farther back than usual. In Squirrels and Marmots their posterior 
extremity reaches a short distance behind and beneath the last molar. In 
Paramys it reached further backward, upward, and externally to a level with 
the crown of the last molar. The jaw in advance of the molars is not. only 
short compared with the usual condition in most known Rodents, but the 
acute edge of the hiatus between the molars and incisors is almost on a 
level with the alveoli of the teeth, instead of forming a deep concave notch, 
so conspicuous a feature of the jaw of the Gnawers generally. The ridge 
defining the muscular fossa on the back part of the jaw is strongly pronounced, 
indicating powerful masticatory muscles.’—( Leidy.) 
