192 THE DESEADO FORMATION OF PATAGONIA 



the sixteen which we found would hardly represent the 

 real proportion of the species in the fauna. 



The jaws are not only small, but also slender and deli- 

 cately built, with the premolar about the same size or 

 slightly larger than the molars. The drawings represent 

 the proportions accurately so I will give but a few measure- 

 ments. 



Specimen Ameghino's 



3009 Type 



Upper premolar 4 to molar 3 8.5 mm. 



Lower premolar 4 to molar 3 95 mm. 



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. 



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- 

 Fig.T 23 . Lower pren^iar 4 posterior diameter greater than the 

 —moiar 2, x 4/1. 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 



