Scotamys gen. nov.

A lower jaw, with premolar 4 and molars 1 and 2, from the Deseado beds on the Chico del Chubut River, west of Puerto Visser, indicates a genus of hystricomorph rodents not previously reported. The lower molars suggest those of Perimys, but premolar 4 is similar to that of Scotaeumys from the Santa Cruz. Scotamys differs, however, from Scotaeumys, in that its molars do not have the third lobe found in the Santa Cruz genus. I have, therefore, made a new genus which appears to be ancestral to Scotaeumys.

Scotamys antiquus sp. nov.

Fig. 125. Lower premolar
4—molar 2, × 4/1.

This, the type species of the above genus, is based on specimen 3063, a lower jaw with the incisor, premolar 4 and the first two molars. The incisor is fairly large and heavy, the anterior face slightly convex, and the anterio-posterior diameter greater than the transverse diameter. Premolar 4 has a deep external fold, dividing the crown into an anterior and posterior lamina, the former being then subdivided by another external fold, making the tooth three-lobed. Just internal to the median fold is a tiny pit, apparently the last vestige of an internal fold. Each molar consists of two laminae separated by a deep external fold, around the inner end of which the laminae are connected by a narrow bar. In the present condition of wear there is no indication of secondary furrows. The premolar is smaller than the molars.

Measurements Specimen 3063
Lower dentition, in. 1 to pm. 48    mm.
Lower dentition, premolar 4 length  3    mm.,  width 2.5 mm.
Lower dentition, molar 1 length2.5  mm.,width 2.5 mm.
Lower dentition, molar 2, length2.75 mm.,width 3   mm.
Height of mandible under pm. 45.5  mm.

Fig. 126. Left mandible external side, × 4/1.