100 MURIDA. 
To the Ropentra belong the greater number of living Mammals, 
and their distribution is practically cosmopolitan, but, as with 
the remaining Orders of EurneEria, they are not well represented 
in Australia, only one of the eighteen recognised Families being 
indigenous. The absence of canines and the large chisel-shaped 
incisors distinguish them from all other Mammals. 
The dental formula is, asa rule, very constant, and may be 
cited typically as I. +, C. $, PM. + or 3, M. 2. 
There are usually five digits in the hand, the pollex, however, 
being sometimes rudimentary or even absent. 
Family I1—MURID. 
Rodents of various habit, but generally terrestrial ; with con- 
tracted frontals ; with the lower root of the maxillary zygomatic 
process more or less flattened into a perpendicular plate. Malar 
short and slender, generally reduced to a splint between the 
maxillary and squamosal processes. Typically with a high per- 
pendicular infraorbital foramen, wide above and narrow below. 
Lower incisors compressed ; molars rooted or rootless, tuberculate 
or with angular enamel folds; premolars none, except in Sminthus,; 
pollex rudimentary ; tail generaly subnaked and scaly. 
The Rats constitute more than a third of the known Rodents, 
and are represented by thirty-five genera with a cosmopolitan 
distribution. The presence or absence of roots to the molars 
divide these into two distinct sections. 
Subfamily I.—Hypromyinz. 
Molars 2, divided into transverse lobes ; infraorbital opening 
variable ; incisive foramina very small. 
Genus I.—HYDROMYS, Geoffroy (1805). 
Molars two only in each ramus, divided into transverse lobes. 
Infraorbital opening crescentic, scarcely narrowed above, its 
external wall without an anteriorly projecting plate. Incisive 
foramina very small. Toes partially webbed. 
Distribution.—Australia ; Tasmania ; New Guinea. 
Dentition.—I. 7, M.4 = 12. 
Habits.—Strictly aquatic, frequenting banks of rivers, creeks, 
water-holes, and inlets of the sea. In their habits they are rather 
shy, and strictly nocturnal or crepuscular. 
Note.—This and the succeeding genus differ from all other 
Rodents in the paucity of their molar teeth. Their food consists 
of mollusks, crustaceans, and other aquatic animals, along with 
vegetable products. 
