THE SHORT-TAILED MEADOW MOUSE 



203 



of these triangles are formed by ridges of 

 hard enamel, while the " prismatic spaces " 

 enclosed within the sides consist of soft 

 ivorv. which, besides maintaining the 

 shape of the tooth, is probably resilient 

 enough to serve as a cushion for the more 

 brittle enamel. The ivory naturally wears 

 hollow, and it is obvious that, when the 

 lower jaw works quickly back and for- 

 ward, a number of cutting edges are simul- 

 taneously brought into play, with the 

 effect of a mincing and grinding machine 

 combined. No better form 

 of tooth could be imagined 

 for dealing quickly with rough 

 marsh-land vegetation. 



Considerable importance is 

 attached to the number of 

 the prismatic spaces as a 

 means of distinguishing one 

 species of Meadow Mouse 

 from another, and if the 

 spaces always composed them- 

 selves into figures which 

 might reasonabh' be called 

 distinct triangles, or indeed 

 chstinct anything, this canon 

 would be practically as well 

 as theoretically useful. The 

 chfhculty. however, of defining 

 a " prismatic space " is shown 

 by the fact that, whereas Con- 

 tinental naturalists appear to 

 recognise as many prismatic 

 spaces as possible, English 

 naturalists appear to recognise as few 

 prismatic spaces as possible. In the 

 case of the lower jaw of the Short- 

 tailed Meadow Mouse, for example, 

 French reckoning gives the number of 

 prismatic spaces in the three teeth as 

 9, 5, 3, whereas English reckoning gives 

 them as 7, 5. 3. 



It would be expected that an animal 

 whose normal habitat was damp would 

 be coated with a fur which would tend 

 to throw off water ; a fur, that is, which, 

 apart from any natural oiliness, would be 

 mainly composed of fine hairs set closely 

 together, and without pronounced spear- 

 tips. 



This is unquestionably the case with the 

 Short-tailed Meadow Mouse and his near 

 relation, the Orkney Meadow Mouse, 

 whereas in the case of the Red-backed 

 Meadow Mouse and the true Mice generally, 

 the tendency for each hair to become spear- 

 tipped is marked. This tendency is 

 accompanied by a redder coloration, 

 owing to the fact that the pigment cells, 

 which, for the greater part of the hair's 

 length, are arranged in a single row, 

 tend, in the spear-tip portion, to break 



THE ORKNEY MEADOW v - 

 Distinguished by its larger and heavier build, shorter relative 

 length of tail, blunter muzzle, and finer hairs. 



up and diffuse their pigment. Measured 

 under the microscope, the average width 

 of the thickest portions of the hairs of the 

 four species works out as follows : — 



Orkney Meadow Mouse 018 mm. 



Short-tailed Meadow Mouse -026 mm. 

 Red-backed Meadow Mouse -052 mm. 

 Long-tailed Field Mouse -045 mm. 



The result of the fineness of the hair 

 in the first two species, with which is 

 associated a brown rather than a red 

 coloration, is that the individual hairs 

 lie close to one another almost to the 

 tips, and thus form a covering which is 

 impervious to ordinary moisture. 



DoLGLAs English. 



