436 U. 8. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



Tribe MURES. 



Molars broad ; tuberculatc ; the tubercles in transverse series. Upper anterior molar, with three tubercles in r 

 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 Mm, to which all of our introduced species 

 belong. The other principalgenera are Cricetus, Cricetomys, PJdoeomys, Hapalotie, Pseudomys 

 Dendromys, and AJcodon. Of these the genus Criceius 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 Ntotoma Hesperomys, or 

 some other peculiarly American genus. 



M US, Lin ri 3d us. 



Mus LINNAEUS, " Systema Naturae, 1735." 



Molars as in other Murinae, f ^ $ ; 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 Un ; ted States, viz: Musdecumanus, 

 rattus, tectorum, and musculus. Mus leucogaster has been found in Brazil. Several species 

 d scribed 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 Mus 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 Merionidcs, 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, 



