Dr. Lantz gives the following brief classification in his section of the publication, "The Rat and Its Relation to Public Health."
Order: Rodentia.
Family: Muridæ.
Genus: Mus.
Species are many, but only three or four are cosmopolitan.
Cosmopolitan species: Mus rattus—black, brown, and roof (Alexandrine) rat; Mus decumanus—gray, barn, wharf, sewer, and Norway rat.
Mus rattus has many varieties known throughout the world and these are named according to color and habitat.
In addition to the names given in Lantz's classification, we constantly see reference to the black house rat, the brownish-gray rat (Mus Alexandrinus), the ordinary ship rat, the field rat, etc.; terms descriptive of habitat and appearance being very loosely applied. Little account is taken, by many, of the well-known variations in the coloration of rats due to climate and season and of the well recognized aptitude of the rat for living in-door or out-door according to circumstances of food supply, weather, etc. The "sawah" rat of Dutch India, implicated in the prevalence of plague there, was formerly considered a variety of Mus decumanus, but is now described as a field variety of Mus rattus. So too, varieties of Mus decumanus are frequently named according to alleged geographic origin, habitat, color and habits, viz.: sewer rat, brown rat, Norway rat and migratory rat.
The inevitable confusion bound to arise from such loose classification is obvious.
Another genus, Gunomys (Nesokia), implicated in plague, is represented in India by two species and by at least one (an undetermined one) in Java, some confusion existing in the matter as yet. Members of this genus are described as large, rough-coated rats which live both as house rats and field rats. In India the Plague Commission reported specimens of this genus as particularly susceptible to plague.