RODENTIA MURINAE. 435 



species. I can, however, hardly assent to the sweeping generalizations of European writers in 

 regard to the superiority of their rats, as a class, to ours. The fact that the Norway rat displaces 

 the native species, whenever brought into contact with them, is easily explained by reason of 

 their superior bulk, and the fact that the American species are much less in the habit of fre 

 quenting the vicinity of dwellings, Furthermore, the black rat, likewise a native of Europe, 

 after it has taken possession of any premises, is displaced and exterminated by the Norway rat, 

 both in this country and Europe, although not much inferior to the latter in size, and, like it, 

 an Old World species. 



There is a third group of Old World Murinae intermediate between the Mures and Sigmodontes, 

 which has no American representatives, and by Wagner termed 



MERIONIDES. The molars are not tuberculate nor pointed, but plane, the crowns traversed 

 by transverse complete lamillae. The indentations of the teeth opposite, not alternate. 



This group is distinguished from that of the true rats by having no tubercles on the molars, 

 the crowns of which are composed of lamellae arranged nearly parallel, one behind the other. 

 When very young, indeed, there may be transverse ridges, but these are continuous, not tuber 

 culate, and, when slightly worn off, exhibit transverse and continuous loops of enamel. Still 

 more worn, there is seen an encircling border of enamel, which indents the tooth, so that those 

 of opposite sides are opposite to each other ; not arranged in a zig-zag manner, with those of 

 opposite sides alternating, as in the ArvicoUnae and most Sigmodonts. The principal genera 

 are Her tones, Rhombomys, Psammomys, Mystromys, Malacotkrix, and Euryotis. All are natives 

 of Africa or the continental portions of Asia. None of these genera occur in America, the so- 

 called Meriones (more correctly JacuLus) belonging to the Dipodinae. 



