RUDENTIA—MURINAE—MUS. 437 
its tubercles are reduced in similar proportion. In the smaller species of Mus, as the mice, this 
tooth is relatively much smaller, and its constituents are only distinguishable with a pretty 
powerful lens. 
The molars of the-lower jaw are comparatively smaller, which permits the division of the 
transverse ridges into only two tubercles, each about equal in circumference to the central 
tubercles of the upper molars. The first lower molar is comparatively longer than the upper ; 
it consists of four transverse folds, or ridges, the posterior very small, and having but one 
tubercle, and lying enclosed in the concavity of the preceding one. The second and third 
ridges are about equal, the first, however, is smaller, and its inner tubercle larger than the 
outer. The second molar has three cross ridges, much like the last three of the first molar ; 
in MW. decumanus, and perhaps in other species, there is, however, a small tubercle on the 
anterior face of the outer anterior tubercle, which may, in fact, be considered as a single and 
rudimentary external element of a fourth cross ridge like the first tooth. This tooth is shorter 
than the one which precedes it, but of even greater diameter ; it is larger than its fellow of the 
upper jaw, in which respect the third molar resembles it; the first lower molar, though 
narrower than the first upper, is longer. The third lower molar consists only of two cross 
ridges, an anterior bituberculate one and a posterior, of considerable size. 
When the teeth are much worn down, the first upper molar is seen to have two transverse 
folds of enamel islands, the following two only one complete one. In the lower jaw, the first 
molar has at first four, but the posterior soon disappears, leaving but three; the second and 
third molars have but one. In teeth of full grown but not old animals, the tubercles only of 
the cross ridges are worn off, and each tooth has as many broad, flat, sharp-edged cross ridges 
as it possessed originally of tuberculate ones. 
The tail in the rats is covered with short, cordiform, quadrate scales, between the successive 
whorls of which appear stiff hairs of greater or less extent and number; upon the varying 
conditions in this respect depend the greater or less amount of covering to the tail. The 
number and degree of continuity of these whorls sometimes furnish good specific characters. 
The tail itself is seldom shorter than the body ; in most instances it is longer. The paws are 
not naked, but the hairs are so short as usually to allow the flesh color of the skin to show 
through. Their nails are tolerably short, pointed, sub-compressed, and moderately curved. 
The palms are naked, with five small balls, three under the four toes; the fourth thumb ball 
is fused with the toe itself; a fifth ball is placed outside on the base of the hand. The balls 
of the hind foot are much more prominent, but similarly placed ; there is a sixth on the inner 
edge of the sole, which is naked. 
The fur consists of hair of different sorts; a softer, shorter, and woolly basal hair, with 
longer and stiffer ones interspersed. In old animals, especially the larger rats, some of the 
latter have a still greater development, especially on the lower part of the back. These hairs, 
when examined with a lens, are seen to be flattened with a fine longitudinal furrow. 
The female has five pairs of teats, three pairs on the belly, between the thighs, and two on 
the breast, between and behind the arms. 
