SClUltlDiE— EXTJNCT SPECIES. 



938 



ARCTOMYS VETUS Marsh. 



Arclomys rtlut MABSit, Aiuer. Journ. 8ci. aud Art% 3il set. ii, 1871, 121. 



About onc-tliird as large as Arctomys monax. Dcscribod from a nearly 

 perfect lower jaw and other rciDains. Lower incisors with a siiallow median 

 groove on the anterior surface. Loup Fork, Northern Nebraska; Tiiocenc. 



Genus PA«AMYS Lcidy. 



raramp Lkidv, Proo. Acad. Nat. Sci. Pliila. 1871, aiil | Ann. Rep, II. S. Geol. Surv. Terr, loi 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 from 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 {J*aramys rohustus Marsh) exceeds in size the common 

 Marmot {Arctomys monax), while the smallest (Paramys relictus Cojie, j)cr- 

 liaps not strictly referable to this genus) is not larger than the common Chick- 

 aree {Sciurus hudsonius). As in all the other genera of the Sciurida, the 

 lower grinding teeth arc four in number on each side, with short, square, 

 tuberculatc, enameled crowns, with the same concave surface and tuberculatc 

 corners as in other genera. 



"The lower jaw is proportionally shorter and deeper than in most known 

 Rodents, Ihe reduction in length being mainly due to i 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 ; I(!vel 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 tlic muscular fossa on the back part of the jaw is strongly pronounced, 

 indicating powerful masticatory muscles.'' — (Leidy.) 



