ALLEN: AMERICAN COLLARED LEMMINGS. 513 



projecting forward from the posterior rim of the orbit. The inter- 

 orbital borders become ridged with age, but the two ridges never 

 meet even in aged individuals but preserve a shallow groove medially. 

 The molar teeth are characterized by their alternating triangular 

 enamel prisms, and by the presence of a small supplementary enamel 

 loop at the posteTO-external end of the first and second upper molari- 

 form teeth, and at the antevo-intenial corner of the first lower molar 

 (mi). 



Two superspecific groups are readily recognizable, and have the 

 value of subgenera. The one is represented by the Labrador Lem- 

 ming, and is the typical subgenus, since Gloger's Dicroston^-x was 

 based, probably, on Pallas's Mus hudsonius (see Miller, 1896, p. 38). 

 The other group includes the remaining living forms of Asia and 

 America so far as at present known. The earliest available name 

 applied to this group is Misothermus of Hensel (1855) who used the 

 term in a generic sense, and particularly specifies Myodcs torquatus 

 as the type, basing his distinctions largely on Middendorf's figures. 

 These two subgenera may be distinguished as follows : — 



DiCROSTONYX : — first and second upper cheek-teeth without a 

 minute accessory enamel fold at the posteTO-infernal corner; both 

 of these teeth terminate with a posteriorly convex transverse outline. 

 Last lower cheek-tooth (m^) normally lacking an accessory antero- 

 internal and antero-external enamel fold. 



Misothermus : — first and second upper cheek-teeth with a very 

 small accessory enamel fold at the postero-intenial corner; the pos- 

 terior outline of the last inner triangle of each tooth concave forward. 

 Last lower cheek-tooth provided with an accessory antero-internal 

 enamel fold, and, in some forms, with an antero-external accessory 

 fold. 



Fossil Collared Lemmings. 



As previously noted, Hensel (1855) figures and describes the upper 

 cheek-teeth of a fossil Collared Lemming from Quedlinburg, Germany, 

 in which according to both description and figure, the typical Dicro- 

 stonyx pattern is present. Since he expressly "states, however, that 

 his figure is not C][uite accurate, the account needs confirmation before 

 this subgenus is admitted in the Old World fauna. Other fossil 

 remains from the Pleistocene of Great Britain, France, and German^', 

 seem to represent Misothermus. Two jaws examined from the 



