RODENTIA MURINAE SIGMODONTES. 445 



T r i b e SIGMODONTES. 



Molars complicated, with the points on the crown never more than two in the transverse series, or rather arranged in 

 two longitudinal series. Confined to the New World. 



The genera which may be grouped under this bead are Hesperomys with its subdivisions, 

 fieithrodon, Holochilus, Sigmodon, and Neotoma, the latter approaching the Armcolinae in the 

 shape of the crowns of the molars. All are found in North America, except Holochilus, and 

 most of the subdivisions of Hesperomys. Neotoma and Sigmodon do not occur south of Mexico. 



In the structure of the incisors, the Sigmodontes do net differ from the Mures ; the molars are, 

 however, constructed on an entirely different pattern. In extreme youth each tooth exhibits 

 two longitudinal series of high conical tubercles, connected by somewhat lower transverse ridges. 

 The first molar has generally three tubercles on each side, the second two, and the third only 

 one, with sometimes a median one on the hinder edge. The first tooth, however, particularly 

 the contracted one of the lower jaw, sometimes has the first or anterior pair of tubercles so close 

 together and reduced as to appear like one, especially when somewhat worn. At the bottom, 

 between the elevated crests connecting these tubercles, may be seen sharply defined curved narrow 

 furrows ; these furrows or grooves (generally darker than the rest of the crown) mark the 

 spaces between the enamel folds, which dip deep into the dentine. The direction and size of 

 these furrows varies on the two sides of the teeth. In the upper jaw they penetrate more deeply 

 from the outside, and bend around a little backwards, so as to be rather convex anteriorly, con 

 cave posteriorly ; in the lower jaw, on the other hand, it is the inner furrow which is deepest, 

 arid this curves forward so as to be concave anteriorly. The furrows of each tooth opposite to 

 those described (the inner of the upper molars and the outer of the lower) are shorter, broader, 

 and more perpendicular to the longitudinal direction of the tooth. In some forms, a very close 

 examination will bring to light a small accessory tubercle in the furrows, near the outer edge, 

 below the level of the others. 



The above structure is that presented by an unworn tooth ; it is, however, rare to find one in 

 this condition, abrasion commencing at a very early age. In time, the tubercles, the most pro 

 jecting part of the enamel, as well as the connecting crests, are all ground off, leaving a flat 

 surface, bounded by a zigzag outline of enamel. After the abrasion is continued still further, 

 the points of the accessory tubercles are ground off, so that behind each salient angle of the 

 enamel zigzag is attached a small accessory angle, and the bend of the curve externally is no 

 longer simple but double ; consisting of a larger and a smaller curvature. Up to this point the 

 narrow folds in the grinding surface are still open externally, but, with increasing abrasion, 

 they become obliterated, or ground out externally, leaving narrow curved enamel islands, on 

 account of being less deep externally than towards the centre of the tooth. This condition of 

 external folds with islands of enamel marks the maturity of the species ; subsequently all the 

 latter are ground out, leaving merely an obtusely indented border of enamel around the tooth 

 substance, the folds dipping in deeper on one side than another. This indicates the old age of 

 the animal. These successive changes do not all take place at the same time in the different 

 species ; but their general progression and succession is much the same in all. 



From the preceding remarks, (by Burmeister,) based on a comparison of a number of skulls 

 of South American species, it will be seen that the number of folds in the teeth will not afford 



