54 



Rats and Mice 



h. Tail prehensile, naked at the tip above. Orifice of ear 

 closed by a conspicuous valve. Mammae in females eight, 

 tv^o pairs on the chest, two pairs in the groin. Size very 

 small (head and body 55 to 75 mm. ; tail 51 to 72 mm. ; hind- 

 foot 13 to 16 mm. ; ear 8 to 10 mm.). 



Harvest Mouse, Micromys minutus. 



B. First and second upper molars with only two tubercles on their 

 inner sides. 



a. Upper incisors (Fig. 3 a) with outer side of cutting edge 

 entire ; front upper molar with five roots, its crown not so 

 long as the combined length of the succeeding teeth, and its 

 structure not peculiar (Fig. 5 D and E). Size large (Rats). 



1. For external characters see p. 1. 



Skull (Fig. 6 D) rather lightly and delicately built ; temporal 

 ridges not parallel, but strongly curved outwards on the sides 

 of the braincase ; the length of a parietal measured along a 

 temporal ridge noticeably less than the greatest width between 

 these ridges ; parietal region with slight but noticeable con- 

 vexity when viewed in profile ; condylo-basal length 38 to 

 45 mm., rarely more than 43 mm. 

 Black Eat, Battus rattus. 



2. For external characters see p. 2. 



Skull (Fig. 6F) strongly built; temporal ridges nearly straight 

 or parallel, never boldly curved outwards on the sides of the 

 braincase ; length of a parietal measured along a temporal 

 ridge about equal to the greatest distance between these 

 ridges ; parietal region flattened, without any conspicuous 

 convexity when viewed in profile ; condylo-basal length 43 to 

 55 mm., rarely less than 45 mm. 



Common Rat, Battus norvegicus. 



h. Upper incisor (Fig. 3 h) with outer side of cutting edge 

 broken by a conspicuous notch and projection ; front upper 

 molar with three roots, its crown longer than the combined 

 length of the two succeeding teeth, and of highly peculiar 

 form (Fig. 5 C). Size small (Mice). 



1. For external characters see p. 37. 



Skull (Fig. 6 E) small and deUcate ; temporal ridges very 

 faintly indicated ; a strongly marked peg-like process of bone 

 on the lower surface of the maxilla immediately below the 

 anterior end of the infraorbital canal; the anterior palatal 



