EXTERNAL CHARACTERS 5 



cotherium; it is the lesson turned to practical use by Winge in 

 systematic work. If the more or less substantial mask of 

 specialization be stripped off from each species one finds the 

 primitive core of each animal underneath; if the primitive 

 characters so found be used as the bases of comparison there is 

 no difficulty either in arranging species, genera, and families in 

 natural order, or in conceiving what the essential characters of 

 the ancestor common to any given group must have been. That 

 is what I have attempted to do in this book. The stripping 

 process is, however, by no means easy, and it reveals many a 

 disconcerting gap in our knowledge. 



c. General Characters of the Microtin^. 

 External Characters. 



In general outward form the Microtina3 differ but little from 

 ordinary rats and mice. All are more or less evidently modified 

 for burrowing, and a few, in addition to their fossorial peculiarities, 

 show conspicuous adaptation for aquatic habits. The members 

 of the subfamily thus display a somewhat striking uniformity in 

 outward appearance, and lack that variety of shape which is so 

 characteristic of the Cricetinee and Murinae. 



In relation to their size voles and lemmings are robust, thickset 

 animals with broad, more or less flattened heads and short, 

 bluntly rounded muzzles. The eyes and ears are small and in 

 some genera are very greatly reduced. The limbs are moderately 

 long, muscular and powerful; but they are hidden to a great 

 extent in the general integument of the trunk, a circumstance 

 which gives the Microtince a characteristic short-legged 

 appearance. 



The hands and feet have each five digits armed with claws 

 which differ considerably in size and form, according to the habits, 

 in different genera ; the thumb is always greatly reduced and bears 

 either a small claw or else a flattened nail. When least modified 

 the palms bear five, the soles six well-developed pads ; but in the 

 more highly speciaUzed forms some or all of these pads may 

 lose their functional importance and tend to disappear. Typically 

 the palms and soles are naked between the pads, or but scantily 

 and incompletely clothed with hair ; but in the more specialized 

 forms they may become either completely naked, as in the aquatic 

 genus Ondatra, or densely covered with hair, as in the boreal genus 

 Dicrostonyx ; in both these genera the modification is accompanied 

 by reduction of the palmar and plantar tubercles. The upper 

 surfaces of the hands and feet are always well clothed, but the 

 lower surfaces of the digits are u-sually naked, with the skin thrown 

 into scaly annulations by transverse folds. 



The tail is never very long, but may reach a length about two- 

 thirds that of the head and body ; in several genera it is quite short 

 and in some is reduced to a mere vestige considerably shorter 



