RODENTIA MURIJTAE SIGMODON. 501 



SIGMODON, Say&Ord. 



Sigmodan, SAY & OBD, J. A. N. Sc. Phila. IV, n, 1825, 352. 

 WAGNER, Suppl. Schreb, III, 1843, 555. 

 AUD. & BACH. N. Am. Quad. I, 1849, 227. 



SP. Cn. General appearance, arvicoline. Ears and tail moderate. Molars rooted, surface plane ; the two last in the lower 

 jaw with the enamel forming a sigrna, or an S- 



In external appearance, the species of Sigmodon closely resemble the larger arvicolas, in the 

 stout body, the long hair more or less concealing the ears, and the tail shorter than the trunk. 

 The under hair is tolerably soft, but there is so great a development of longer hairs as to 

 produce quite a hispid appearance, whence the name of one species. 



The muzzle is very blunt, as in Arvicola, the ears are large, but much buried in the fur ; the 

 muzzle hairy, except on the septum. The upper lip is emarginate, but not very deeply 

 cleft. The antitragus is large and valvular. The thumb is rudimentary, covered by a broad, 

 flat, nail. In the hind foot the lateral toes (1st and 5th) are very short, and nearly equal, 

 their claws reaching barely to the bases of the adjacent toes, instead of projecting considerably 

 beyond them, as in Neotoma, Hesperomys, Mus, &c. The soles are perfectly naked from the 

 very heel, and exhibit six black tubercles much smaller than in Neotoma or Hesperomys ; the 

 skin around these tubercles is granular, very different from Mus. The claws are longer than 

 in Mus or Neotoma. 



The incisors are rather stout and a little compressed ; the upper ones are much rounded on 

 the anterior face which has, besides, one or two faint ridges. Owing to the convexity transversely 

 of these incisors, a considerable portion of the anterior face is visible from the side. The lower 

 incisors are about as wide as the upper, and bevelled off on the outer edge so as to be triangular 

 in cross section. 



The anterior upper molar is the largest, the second and third diminishing successively. The 

 second is as broad as long, as wide or wider than the first ; the third nearly as long, but narrower. 

 The enamel folds have the same general characters as described for the sigmodonts, (viz : their 

 convexity posteriorly above and anteriorly below, deeper indentation on the outside above, &c.) 

 All the upper molars have two indentations or re-entrant loops of enamel on the outer side ; on 

 the inside, the first has two, the remainder have each one. There are nowhere on the folds 

 any acute angles with straight sides, but all are regular rounded loops. The first and second 

 inner loops of the first molar are a little anterior, respectively, to the first and second on the 

 outside ; on the second and third the anterior outer loop is a little anterior to the single inner 

 one, but so near that they appear in one nearly continuous slightly oblique transverse line. 

 Theloops are greatly indented, so much so that those of opposite sides meet, and the posterior 

 outer single ones extend across to the opposite side, thus dividing the dentine into three or 

 more nearly complete islands. 



In the lower jaw, the first molar is the longest, widest behind and tapering forwards ; th 

 two others are about as broad as long, similarly shaped, the second rather largest ; in fact, 

 somewhat wider than the first. The first molar has three indentations or re-entrant loops of 

 enamel on the outside, and two inside, the others have one only on each side ; here the inne? 

 loop is anterior instead of the outer, as in the upper jaw ; those of opposite sides considerably 

 separated. The folds are indented as much as in the upper jaw. The second lower molar, in 



