GLIRES OF THE SANTA CRUZ BEDS. 421 



ACAREMYIN^. 



Ameghino has proposed this subfamily to include Acaremys and 

 Sciamys, two of the smallest and most abundant of the Santa Cruz 

 rodents. The propriety of marking the distinction between these genera 

 and the Erethizontincz is evident, but it may well prove to be the case that 

 the group should be raised to family rank, for the resemblance to the other 

 subfamily lies principally in the dentition, while in the skull and skeleton 

 there are significant differences. 



SCIAMYS Ameghino. 



(Plates LXVI, Figs. 10-11"; LXVII, 1-3, 5-9.) 



Sciamys Amegh. ; Enumeracion sistem, etc. ; 1 887, p. 9. 



Small rodents with low-crowned and rooted grinding teeth, much 

 resembling those of Steiromys in pattern ; the crown is divided into two 

 lobes by opposite enamel folds, and each lobe has a secondary fold, or 

 enamel lake, which disappears relatively early on wear. In both jaws the 

 two series of grinding teeth are nearly parallel, or have a slight anterior 

 divergence. The incisors vary considerably in the different species, but 

 are usually very narrow and delicate ; in the upper jaw, they may have 

 either plane or slightly convex faces, but in the mandible they are always 

 plane, except in S. robustus, in which they are feebly grooved. The lower 

 incisors are very long, extending behind the molars into the ascending 

 rami. 



The skull and skeleton are still very incompletely known, but some 

 significant facts have been ascertained. In its general appearance and 

 proportions the skull is much like that of the recent Echimys, though with 

 much less curved upper contour. In the only specimen that I have seen 

 which retains the posterior portion of the cranial roof (PI. LXVII, fig. 9), 

 there is no trace of sagittal crest or area, though the individual was fully 

 adult. The occiput is broad and low, gently convex in the middle, with 

 very large foramen magnum and small, sessile, widely separated condyles ; 

 the paroccipital processes are small and well divided from the bullae. 

 As in this family generally, the supraoccipital has quite long lateral 

 processes, which extend down on each side upon the periotics. The tym- 

 panic bulla is largely inflated, with auditory meatus in the form of a very 

 short tube, less prominent and less complete dorsally than in Erethizon. 



