454 
U S. P. R. R. EXP. AND SURVEYS-ZOOLOGY-GENERAL REPORT. 
The following features may serve to characterize the above sub-genera, the last of which at 
least, is probably entitled to higher rank. 
Calomys, Waterhouse. 
Calomys, Waterhouse, Pr. Zool. Soc. Lond. V, 1837, 21. 
Wagner, Suppl. Schreb. Ill, 1843, 524. 
Burmeister, Thiere Bras. I, 1854,168. 
Eligmodonlia, F. Cuv. Ann. des Sc. Nat. 1837,168. 
Form highly murine. Tail as long or much longer than the body. Eyes and whiskers large. Upper border of the orbit 
sharp, extending backwards as a ridge along the side of the cranium. Indentation of the enamel of the molars consider¬ 
able ; posterior molar moderate. 
The species of this section are very mouse-like, both in shape and size, although almost always 
white on the belly, feet and under surface of the tail. The fur is long and very soft, with still 
longer hut soft hairs interspersed. The head is pointed, the ears large and coated with very 
short hairs. The elongated tail is coated with short hairs, so as usually more or less to conceal 
the annuli. The hind feet are long and generally correspond in this respect with the tail, both 
being longer or shorter together. 
The skull is rather full, with the nose elongated. The upper border of the orbit is sharP 
and continuous, with a blunt ridge, which passes back along the junction of the temporal and 
parietal bones to the occipital. The enamel folds are rather narrow and parallel sided, and the 
teeth generally exhibit the characters described under Sigmodontes. 
Habrothrix, Waterhouse. 
Habrothrix, Waterhouse, Pr. Zool. Soc. V, 1837, 25. 
Wagner, Suppl. Schreber, III, 1843, 516. 
Burmeister, Thiere Bras. I, 1854, 178. 
Form arvicoline. Eyes and whiskers small. Tail and feet short; the former with short hairs. Ears well furred. Skull 
elongated. Edges of orbit rounded. Indentations of enamel short. Posterior molar very short. Thumb with a nail. 
The species of the sub-genus Habrothrix bear a close external resemblance, except iu the sharp 
snout, to the Arvicolas, and, in fact, would seem to represent them in South America. The 
eyes are small. The ears are quite large, broader than high, and well covered with hair. The 
muzzle is long and pointed ; the whiskers short and scanty. Tail shorter than the body, rather 
densely coated with hair. Tail and feet shorter than in Calomys. Thumb of fore foot with a 
broad flat nail, instead of a claw, as in Oxymicterus. 
The skull is narrow and elongated, the muzzle especially so ; the upper edges of the orbit not 
sharp, but rounded off. The malar bone is shorter and less curved than in Calomys. The 
lower incisors are narrower ; the posterior nearly circular upper molar is decidedly smaller. 
The enamel folds penetrate less deeply, and the molars, when half worn, do not exhibit any 
isolated enamel islands on the grinding surface. The fur is soft, the longer hairs projecting 
less than in Calomys , and the color is of a more homogeneous grayish. 
Scapteromys differs in having a claw to the thumb, all the fore claws long, the molars also 
slightly different. 
very close, extending apparently to the very pattern of the enamel' loops; an appreciable distinction, however, is to be 
found in the comparative lengths of the molars. 
No species of Holochilus have yet been detected in North America. Its affinities appear very close with Sigmodon. 
