16 



movements. None of the family have premolars; and, except in 

 a single genus (Hydromys), the number of molars is three. Oldfield 

 Thomas, the eminent English zoologist, includes in this family no 

 less than 77 genera, or almost half the total of 159 which he ascribes 

 to the whole order Rodentia. a He further subdivides the Muridse 

 into a dozen subfamilies, of which the Murinse and the Sigmodontinse 

 are the most extensive. The name Cricetinx is now generally used 

 instead of Sigmodontinse, though not always with the same limitations. 

 The Murinse comprise only Old World rats and mice, while the 

 Cricetinse are, in the main, American .forms. In the Murinx the 

 cusps, or tubercles, of the unworn upper molars are arranged triserially , 

 or in three longitudinal rows; in the Oricetinse they are arranged 

 biserially, one row on the outer and one on the inner margin (fig. 1). 

 The wearing away of these cusps leaves characteristic curved lines 

 of hard enamel surrounding areas of dentine. When the cusps are 



Fig. la. Fig. lb. 



FIG. la. Upper molars of the brown rat (Mus): tubercles in three rows. 

 FIG. lb. Upper molars of the rice rat (Oryzomys): turbercles in two rows. 



in pairs the worn pattern looks somewhat like the Greek letter 

 sigma (JT), whence the name sigmodont, often applied to native 

 American rats and mice. 



The MurinsB are the true rats and mice, typified by the genus JI///.S-, 

 which contains by far the largest number of species. Trouessart, 

 in his Catalogus Mammalium, enumerates 260 species of Mus de- 

 scribed before 1905. Since that date a number of new forms have 

 been described. 



The genus Mus is characterized by narrow, ungrooved incisors; 

 three small, rooted molars; soft fur mixed with hairs, sometimes with 

 spines; a rudimentary pollex having a short nail instead of a claw; 

 a long tail bearing rings of overlapping scales and often naked or 

 nearly so. The ears are rather large, the 63^3 bright and prominent, 



oProc, Zool. Soc., pp. 1012-1028, 1896, 



