502 U. Ss. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
one specimen, (1667,) has a small accessory peninsula of enamel on the inside, which almost 
indicates a second loop ; it is, however, slight, and must disappear at quite an early age. 
The skull, in some respects, has a not inconsiderable resemblance to that of Mus decumanus, 
especially in the well defined, compressed, elevated ridge which margins the upper border of 
the orbit, and extends backwards along the temples, until it bends off downwards nearly at a 
right angle. The head is, however, much broader across the zygomata, and shorter, the muzzle 
wider. The nasal process of the maxillary extends back behind the nasal and lachrymal bones; 
the thin plate at the anterior end of the zygomatic arch is broader, thinner, extends further 
forward in a more nearly vertical plane, and is produced above into a point, with a broadly 
rounded emargination below it. The zygomatic process of the temporal stands out nearly 
perpendicularly. The malar bone is very short, The incisive foramina are much larger, 
extending behind to between the anterior extremity of the first molars. The palato-maxillary 
suture is between the line of the posterior end of the second molars, instead of the first, as in 
the Norway rat; the palatine bone itself is curiously excavated on each side of a median crest, 
with two foramina at the bottom of each excavation. 
The lower jaw has the ramus quite convex externally, the posterior end of the incisor 
protruding on the outside, just below the notch between the coronoid and condyloid processes 
and high above the last molar, instead of terminating just behind it, as in the Norway rat. 
The species of Sigmodon are confined to the southern portions of the United States—from 
Carolina, westward, to western Texas. 
SIGMODON HISPIDUS, Say & Ord. 
Cotton Rat. 
Sigmodon hispidum, Sax & Ory, J. A. N. Sc. Phila, IV, 1, 1825, 354; pl. x, f. 5—8, (read March, 1825.)—Inz. Zool. 
Jour. II, 1825, 296; pl. x, f.5—8. 
Waener, Suppl. Schreb, IIT, 1843, 556. 
Aun. & Baca. N. Am. Quad. I, 1849, 229; pl. xxx. 
Arvicola hispidus, GopMan, Am, N. H. II, 68. 
Arvicola hortensis, Haruan, F. Am. 1825, 138.—Is. Med. and Phys. Res. 1835, 49. 
? Arvicola ferrugineus, Hartan, Am. Jour. Sc. X, Feb, 1826, 285.—Is. Med. and Phys. Researches, 1835, 57. 
Sp. Cu.—Tail less than the trunk. Color above, reddish brown, lined with dark brown. Claws very strong. 
This species is about half the size of a Norway rat, Mus decumanus. The head is rather 
blunt and compressed. The ears are large, nearly orbicular, the antitragus much developed, 
covered on both sides with short hairs, except around the auditory aperture, which is naked. 
The tail is shorter than the body, exclusive of the head, covered with short stiff appressed 
hairs, which do not conceal the annuli, and with a very slight tuft atthe end. The third finger 
is longest, the fourth not appreciably shorter; the claw of the fifth reaches a little beyond the 
ante-penultimate articulation of the fourth finger ; that of the second to the end of the ball of 
the third. The central three toes are equal and much the longest. The tips of the claws of 
the first and fifth are about on a line; but the first claw being considerably the larger, its toe 
is shorter, though set further back. Soles entirely naked from the heel. 
The prevailing color of the upper parts is a reddish or rusty brown, varying a little in tint, 
and lined with dark brown; the sides are brightest, and their color extends on the outside of 
the limbs to the fingers of the hand and the tarsus. The under parts generally, with the 
