510 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 92 
the first lower cheek tooth is relatively wider than in S. medius, 
resembling the modern form in this respect. Gidley noted that the 
cheek teeth were less hypsodont than in the living species, that the 
valleys of the reentrant folds were more widely open and the lophs 
more compressed. Although these characters are evident, the extent 
to which they are exhibited is not so noticeable as in S. medius. Asin 
S. medius the posterior reentrant on the lingual side of the second 
tooth is much deeper than in Recent Sigmodon and the posterior por- 
tion of the lingual wall of the third tooth makes a sharp right angle 
with the enamel of the lingual reentrant; however, the lingual wall 
of this tooth does not appear to be notched as it is, though weakly so, 
in the type of S. medius. 
The upper dentition added to the collection in 1936 is in an early 
stage of wear, and the teeth appear noticeably more hypsodont than 
the upper teeth belonging to the type of S. medius. In this respect 
they approach more closely teeth in modern species, although the re- 
entrant valleys appear more widely open than in these. The anterior 
lobe of the first upper cheek tooth, unlike that in the lower dentition 
of S. curtisi, is narrower than in modern material observed, approach- 
ing more closely the relative proportions seen in S. medius. The 
upper teeth of S. curtisi are otherwise similar to those in the modern 
cotton rat, particularly S. sanctae martae of Colombia. 
SIGMODON MINOR Gidley 
A small species of Sigmodon is represented in the Pleistocene col- 
lection by no less than ten lower jaws and a maxillary portion. Four 
of the lower jaws were obtained by the 1936 expedition. The type, 
a left ramus, No. 10512, and seven of the referred jaws include all the 
cheek teeth. 
Sigmodon minor is considerably smaller than its contemporary, 
Sigmodon curtist, but less widely separated from the earlier S. medius. 
There is an appreciable variation in the size of the jaws and teeth, and 
although size of teeth is not in every case correlated with size of jaw, 
one heavy jawed specimen, No. 16611, has slightly larger teeth than 
the smallest of the Benson lot; however, the average of the teeth is 
distinctly less and all of the S. minor jaws exhibit relatively narrower 
teeth than in S. medius. The degree of hypsodonty and the pattern 
of the lower teeth of S. minor correspond closely to those in S. medius 
and the two forms differ in nearly the same respects from the living 
species. 
The upper teeth in No. 10513 are also smaller than those belonging 
to the type of S. medius, and the anterior lebe of the first is a little 
narrower, as noted in several, but not all, of the lower dentitions. 
Gidley noted also that the external reentrants in the first upper tooth 
