134 



THE NATURE BOOK 



case with mice, the hollow of the eye 

 is hardly distinguishable from the hollow 

 of the temple, and both lie back within 

 the circuit of the zygomatic arch, the 

 solidity or otherNN-ise of the latter is of 

 considerable importance in determining, 

 so far as outward inspection can go, the 

 solidity or otherwise of the skull itself. 



HEADS OF THE SHOKT-TAILED MEADOW MOUSE (1) 



MOUSE (2) ; AND LONG-TAILED FIELD 



The shape of the zygomatic arch is a reliable guide in dist 



In the Long-tailed Field Mouse the zygo- 

 matic arch describes a gentle sweeping 

 curve, with no thickening of the middle 

 portion, and but a slight angularity at the 

 hinder end. In the Short-tailed Meadow 

 Mouse the arch presents abinipt, strong- 

 looking angles, and a stout, flattened joint in 

 the centre where two of the bones compos- 

 ing it overlap. In the Red-backed Meadow 

 Mouse there is a midway development both 

 of the joint and of the angularity. 



The lower jaws of the three species 

 afford a similar scale of comparison : 

 in the Long-tailed Field Mouse graceful 

 lines, in the Short-tailed Meadow Mouse 

 soHd ridges, and in the Red-backed 

 Meadow Mouse a mean between the two. 



The teeth of the three correspond with 

 their several settings. It will be con- 

 venient to leave the grinding teeth of 

 Meadow Mice, and the extremelv beauti- 

 ful patterns which their grinding surfaces 

 present, for consideration in a subsequent 

 article, and to confine ourselves for the 

 moment to their incisors or cutting teeth. 



The incisors (front teeth) of Rodents 

 present such unique characteristics that 



it is possible to distinguish a member of 

 the order by them alone. 



They are large, strong, curving teeth, 

 often of a yellowish or orange colour, which 

 grow continuously during the animal's 

 hfetime. The working part is composed 

 of the usual ivory and enamel ; but the 

 enamel, instead of covering the whole 

 exposed portion, is 

 almost if not en- 

 tirely confined to 

 the outer surface. 

 Owing to its hard- 

 ness it wears away 

 more slowly than 

 the ivorj^ behind 

 it, and the result 

 is the formation of 

 a chisel edge, and 

 its maintenance in 

 a sharp condition 

 b y wear and 

 growth combined. 



Both upper and 

 lower incisors grow 

 in regular curves, 

 and, should one be 

 accidentally broken, 

 the opposing tooth, 

 the case may be, 

 continues its curve, and grows back upon 

 itself, wTth disastrous and often fatal 

 results to the animal. 



The upper incisors being situated 

 in the fixed upper jaw are, of course, im- 

 movable. In gnawing, the work is effected 

 mainly by the lower incisors ; and the 

 lower jaw, while admitting of movement 

 in several directions, is specially modified 

 for back-and-forward chiselling. To this 

 end the hinder portions of it are lengthened, 

 giving additional scope for the lengthening 

 and contraction of the masseter muscles, 

 which are attached to the fore-parts of 

 the animal's face and do most of the work 

 in pulling the lower jaw forward. More- 

 over, the portions of the skull to which the 

 lower jaw hinges arc grooved to assist a 

 back-and-forward motion. 



One would expect to find that teeth 

 which acted as chisels, and in which it was 

 extremely important that the edges should' 

 wear evenly, would not admit of any side 

 or cross moVement such as would tend 

 to blunt their edges on the edges of the 

 teeth opposed to them. 



; RED-BACKED MEADOW 



MOUSE (3). 



inguishing the three species. 



above or below as 



