436 U. Ss. P. R. R. EXP, AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
Tribe—MURES. 
Molars broad; tuberculate; the tubercles in transverse series. Upper anterior molar, with three tubercles in each 
transverse series or with the tubercles in three longitudinal series. Confined to the Old World. 
Although all the members of this section are inhabitants of the Old World, yet from the 
number of species introduced into North and South America, and the importance of clearly 
understanding the differences of the two groups, I shall describe their characteristics at con- 
siderable length, basing my remarks upon the genus Mus, to which all of our introduced species 
belong. The other principalgenera are Cricetus, Cricetomys, Phloeomys, Hapalotis, Pseudomys 
Dendromys, and Akodon. Of these the genus Cricetus is distinguished from Mus by the internal 
cheek pouches and hairy tail; has been assigned by some authors to North America, but further 
examination has, in all cases, shown the supposed species to belong to Neotoma Hesperomys, or 
some other peculiarly American genus. 
MUS, Linneus. 
Mus Linnxvs, ‘‘ Systema Natur, 1735.” 
Molars as in other Murinae, 3=3; those of opposite sides paralled to each other; with two or three transverse series of 
tubercles, three in a series longitudinally ; the central series elevated. No cheek pouches. Upper lips divided; snout 
acute, hairy to the fissure, whiskers in five series. Ears large, prominent, nearly naked. Tail long, the scaly whorls very 
distinct. 
For most of the general considerations here presented respecting the minute features of the 
European and American mice I am indebted to Burmeister, who, in his ‘‘ Naturgeschichte der 
Thiere Brasiliens, part I, 1854,’’ has given the best view I have seen of the subject. 
The number of species of true Mus is very considerable, amounting to over fifty. Of these 
only four have hitherto been detected within the limits of the United States, viz: Mus decumanus, 
rattus, tectorum, and musculus. Mus leucogaster has been found in Brazil. Several species 
described from South America, by Waterhouse and others, are probably varieties of some of the 
Old World species, wherever they are really not of American types. Such is Jus Jacobiae, 
Waterhouse, from the Gallipagos. 
The molars of the genus Mus, as already stated, are broader and stronger than in the other 
allied genera. In the unworn condition, each one has two or three transverse ridges, cut up 
by depressions into small tubercles. They are coated externally by enamel, which sends folds . 
into the dentine from both sides, shown in the worn tooth as indentations opposite to each other, 
with islands of enamel between, or even strips of enamel passing entirely across. The enamel 
folds on opposite sides never alternate with each other, but always stands in simple transverse 
series, a little convex anteriorly. In this point of having the indentations of opposite sides 
opposite each other there is a close approach to one of the characters of Merionides, which thus 
becomes applicable to all the Old World Murinae. 
In the upper jaw the first molar has three transverse ridges, each one somewhat curved and 
concave behind. The first and second are divided into three tubercles ; the third has but two; 
the smaller internal one being wanting. The second upper molar has likewise three ridges, the 
anterior, however, has but one tubercle (the inner;) the second ridge has three, the third, 
again, only two, the external one much smaller than the corresponding one on the first tooth. 
The third tooth is quite similarly constituted ; as, however, it is much smaller than the second, 
