430 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



ness not exceeding the width so much as usual, and each one is marked 

 by a groove, shallow and inconspicuous and yet quite distinct ; the mesial 

 border is raised into a fine, almost hair-like ridge, while the outer border 

 is a wider and more distinct ridge. 



MEASUREMENTS. 



Lower incisor, width 002 Diastema, length 0115 



" " thickness 0025 



ACAREMYS Ameghino. 



(Plates LXVI, Figs, u, 11; LXVII, 4-4", 10-13. 



Acaremys Amegh. ; Enumeracion sistem., etc.; 1887, p. 9. 



Closely allied to Sciamys, though not so well known, because it is much 

 less abundantly represented in the collections. The grinding teeth have 

 the same pattern as in Sciamys, but the incisors, both upper and lower, 

 have convex faces and, in most of the species, are quite broad. The 

 skull also resembles that of Sciamys, with a few differences of detail, 

 which appear to be constant. The dorsal branch of the maxillary zygo- 

 matic process is much more slender ; except in A. minutus, which has a 

 process from the jugal, the frontal has a short postorbital process, which 

 projects more abruptly than in those species of Sciamys which have the 

 process. In the parietal zone Acaremys has a distinct sagittal crest, which 

 is not the case in Sciamys. No part of the skeleton other than the skull 

 has yet been found. 



ACAREMYS MURINUS Ameghino. 



(Plates LXVI, Figs, 11, n; LXVII, 4-4*, 12.) 



Acaremys murimts Amegh. ; Enumeracion sistem., etc. ; 1887, p. 9. 

 Acaremys messor Amegh. ; Contrib. al Conoc. de los Mam. Fos. de la 



Repub. Argent; 1889, p. 126. 



This species may be distinguished by its broad and blunt rostrum, 

 almost without anterior taper. To it I refer a specimen in the American 

 Museum of Natural History ( No. 9,280) which unfortunately, has no 

 mandible associated with it and may possibly belong to Sciamys latidens, 

 which it closely resembles, but the upper incisors are broader and more 

 strongly convex, while the part protruded from the alveolus is much 

 longer and the abraded surface much more gently inclined ; the rostrum 

 has more nearly parallel sides and the anterior end is blunter ; the upper 



