100 MURID.E. 



To the RODENTIA belong the greater number of living Mammals, 

 and their distribution is practically cosmopolitan, but, as with 

 the remaining Orders of EUTHERIA, 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, as a rule, very constant, and may be 

 cited typically as I. , C. , PM. i or -, M. f . 



There are usually five digits in the hand, the pollex, however, 

 being sometimes rudimentary or even absent. 



Family I. MURIDJE. 



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 Sminthns; 

 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. HYDROMYINJE. 



Molars t>, 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. \, M.| == 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 niollusks, crustaceans, and other aquatic animals, along with 

 vegetable products. 



