124 



NORTH AMERICAN. FAUNA 66 



Distribution in Maryland. — Occurs abundantly as a commensal or 

 as a feral animal in all sections of the State. 



Distinguishing characteristics. — This small mouse is well known 

 and needs no extensive description. The upper molar teeth of the house 

 mouse are essentially like those in Battue rattus and R. norvegicus^ 

 that is, with three rows of longitudinally arranged cusps. This distin- 

 guishes the species from all other Maryland mice of small size. 



Externally, the house mouse superficially resembles American mice 

 of the genera Peromyscus and Reithrodontomys. It differs from Mary- 

 land Peromyscus externally in its smaller size and in coloration. In 

 adult pelage, Peromyscus is generally a brownish gray in coloration 

 on the dorsum, with a white venter, the line of demarcation between 

 the two being sharply marked. The tail also is distinctly bicolored, 

 darker above, pale below. In the house mouse, the coloration is more 

 grayish and the abdomen is generally paler than the dorsum, but there 

 is no sharp line of demarcation between the two, the abdomen seldom 

 being pure whit« as in Perorruyscus. In addition, the tail of the house 

 mouse is not distinctly bicolored. 



The dorsum of the juvenile Peromyscus is colored a uniform slaty 

 gray, which is totally unlike the grizzled gray of the house mouse, and 

 the venter is a snowy white as in adults, with a sharp line of demarca- 

 tion between the two. 



Externally, the house mouse is very smiliar in appearance to the 

 harvest mouse {Reithrodontomys humulis) . The most certain way of 

 separating the two species is through an examination of the upper 

 incisor teeth. In Reithrodontomys there is a longitudinal groove which 

 runs the length of each incisor, while in the house mouse these teeth 

 are smooth. In addition, the biting edges of the upper incisor teeth of 

 the house mouse usually are notched, and the tips of the lower incisors 

 fit into the notches when the jaws are closed. 



Measurements. — Seven adults from the vicinity of Ocean City, 

 Worcester County, have the following external measurements : Total 

 length 149.3 (140-167) ; tail vertebrae 73 (58-88) ; hind foot 17.4 

 (16-18). Cranial measurements of five adults from the vicinity of 

 Ocean City are: Greatest length 20.9 (19.8-21.8) ; zygomatic breadth 

 11.0 (10.6-11.5) ; interorbital breadth 3.4 (3.3-3.6) ; length of maxil- 

 lary toothrow 3.2 (3.1-3.4). 



Habitat and habits. — This is a very plastic animal, and it has 

 adapted itself to a wide variety of habitats. Like the black rat and 

 the Norway rat, it is most often encountered in or near human habita- 

 tions, but is also found in the wild throughout Maryland. 



