MYOPUS 179 



equalling or exceeding that of the diastema. In the rapid 

 posterior divergence of the tooth-rows, the structure of the 

 hinder part of the palate, the choanse, pterygoid fossae, and 

 basis cranii, as well as in the shortening of the post-glenoid 

 region, Myopus closely resembles Lemnnis. The auditory bullae 

 are, however, more globular, like those of Syiutjjfomys in shape, 

 but smaller and less closely approximated below: their internal 

 structure is essentially as in iSyiiaptoitiys, but the walls of the 

 bullae are more densely spongy and the trabeculae passing between 

 them and the petrous jjortion are finer and more numerous ; the 

 canaliculus tympanicus is very slender where it passes through 

 the stapes, but it apjjears to be ossified at this point as elsewhere. 



Mandible and dentition essentially as in Lemmus; incisors 

 relatively slender, their enamel pale yellow ; upper incisors 

 " orthodont," their anterior faces nearly flush with the tips of 

 the nasals. Cheek-teeth exactly like those of Lejiumis in pattern ; 

 but m^ usually shorter and broader in relation to m^ and «i^, 

 its elements more crowded antero-posteriorly ; in 5/(3 the outer 

 infold occasionally fails to close off an external triangle, a varia- 

 tion recalling the subgenus Mictoiuys but never seen in Lemmus. 



Geographical differentiation. — Our knowledge of this interesting 

 and well-marked genus has increased greatly in recent years. 

 Originally described from Scandinavia it was next detected by 

 Middendorfi on the coast of the Sea of Okhotsk, a circumstance 

 which led both Middendorfi and Lilljeborg to conjecture that 

 the range of these peculiarly rare lemmings extended right across 

 northern Euroi)e and Asia. Middendorfi's record was, however, 

 generally doubted or ignored until 1912, when Hollister described 

 a species M. morulus from the Altai. A little later I described 

 another (M. saianicus) from the Syansk Mountains 600 miles 

 further east, and in the same year, 1914, G. M. Allen gave an 

 account of yet another species {M. thayeri) inhabiting North-east 

 Siberia. Quite recently Vinogradov has re-examined Midden- 

 dorfi's original material from the Sea of Okhotsk together with 

 other specimens since obtained in N. Amur Land and on the 

 eastern shore of Lake Baikal ; and upon the basis of this material 

 he has established his M. ■middendorffi. The same observer has 

 also found that a fossil lemming, of which skeletons and soft 

 parts frozen in the ice-bound soil of a cave in Northern Siberia 

 were found by Tscherski, who origuially described them as Myodes 

 hrandti, is a member of the jn-esent genus. 



The characters separating the six forms now recognized are 

 comparatively slight, relating chiefly to the greater or less develop- 

 ment of the mantle, the size of the skull, the size and degree of 

 inflation of the auditory bullae and the strength of the incisors 

 and cheek-teeth. On the whole M. saianicus appears to be the 

 most sjiecialized form. Allen remarks that " it seems likely 

 that the Altai and the Syansk forms must be very closely related, 

 and both perhaps hardly more than subspecies of M. schisticolor 



