192 MICROTIN^ 



consists of a posterior loop and three alternating triangles of 

 which the first and third are internal; this tooth is essentially- 

 similar to m^, but the fourth triangle or third outer salient angle 

 is reduced and obsolete, and the first inner infold is transverse 

 and nearly as deep as the first outer infold in m^ and m^ above, 

 producing similar though less marked effects upon the form of 

 the first outer salient angle and upon the relation of the posterior 

 loop with the triangle in front. 



Geographical differentiation. — At the present time twelve 

 forms, referred to ten distinct species of Lemmus are recognized 

 by systematists ; but the characters distinguishing them are 

 far from being satisfactorily known. The genus indeed is a 

 very difficult one to deal with. All the forms are very closely 

 related and vicarious. For the most part they have been 

 imperfectly described from scanty and often unsuitable material. 

 No single form, not even the Common Norwegian Lemming is 

 properly represented in our collections. In view of the abund- 

 ance of specimens of L. lemmus this last statement may seem 

 surprising, but it can be justified in a few words. 



Like so many other Microtines the members of this genus 

 grow almost indefinitely ; and they acquire sexual maturity and 

 adult-looking coats at a very early age. In ordinary years they 

 are not easily caught; but in " Lemming years " it is only too 

 easy to collect long series of specimens — of a sort. In such long 

 series the individuals are hardly ever more than adolescent. 

 Owing to the circumstance that in this genus the temporal 

 ridges are at the moment of their inception rather closely 

 approximated in the interorbital region, the unsatisfactory 

 nature of our abundant material is not at first sight apparent; 

 because the ridges meet and fuse to form a weak interorbital 

 crest much sooner than they do, for example, in Arvicola, and 

 the skull therefore acquires an adult appearance long before the 

 Lemming is really grown up. In all the long series of skulls of 

 L. lemmus that I have examined there is but one (B.M. 7.7.7.3260 

 Kola Peninsula ; ex MiUer Collection) that can be regarded as 

 tolerably adult ; and that a skuU even in that stage of growth may 

 be greatly modified with advancing age will be readily apparent 

 from an inspection of PL VI, where it is compared with a frag- 

 mentary but mature skull of L. obensis from the island of 

 Waigatsch. The latter specimen, which I owe to the generosity 

 of the late Dr. Forsyth Major, transformed as it is by long-con- 

 tinued growth has not yet reached the limit of possible develop- 

 ment ; its sutures are still open, although they have now acquired 

 the tortuous courses characteristic of late stages of growth, 

 and the dentinal pulps and enamel organs of the cheek-teeth 

 were stiU vigorously active at the moment of the animal's death. 

 Comparing such a skull with those " fine series " upon which 

 the numerous species described in recent years have been based, 

 we find it not difficult to appreciate why in this genus the skull 



