DICROSTONYX 159 



Arvicola (Myolemmus) ambiguus, leaves no room for doubting 

 that Myolemmus is a synonym of Dicroslonyx. Unfortunately the 

 lower jaw, in this genus, yields little that is diagnostic of species, 

 and Pomel's description contains nothing that will enable us to 

 determine which of the fossil species should bear the name 

 ambiguus. The description of the lower jaws is followed by an 

 account of a skull which Pomel referred also to A. {M.) ambiguus ; 

 but from the characters mentioned it is evident that this skull 

 belonged to some other genus — probably Lemmus. Pomel's 

 species '" amhiguus " therefore cannot now be identified and the 

 name must be dropped. 



In 1855 Hensel ^ described a fragmentary skull from a Pleis- 

 tocene deposit at Quedlinburg, Saxony. Possessing at that time 

 no recent material and knowing the skull of the recent animal 

 only from the poor figures given by Middendorff, Hensel recog- 

 nized the close affinity of the fossil species with the living D. tor- 

 qiialus. He also appreciated the great differences which exist 

 between the skull and teeth of the Arctic Lemming and those of 

 the Norwegian Lenmiing and its allies, and he separated the 

 former as a distinct genus Misothermus from the latter, which he 

 left in the genus Myodes (= Lemmus). A little later Hensel 

 procured a skull of D. torquatus from the Taimyrland, and found 

 that the two anterior upper molars {m^ and /u') had each a minute 

 postero-internal angle which was not present in the teeth of the 

 Quedlinburg fossil. ^ But reasoning from his experience of the 

 variability of the posterior ends of the upper molars in voles 

 generally, Hensel justly remarked that this minute dif?erence 

 must be held to be a mere individual peculiarity until such time 

 as one could examine a sufficiently large number of dentitions 

 to prove the contrary. 



In 1870 Sanford^ referred a skull from the Hutton Cave, in 

 Somersetshire, to Lemmus torquatus ; but he based a new species 

 of Arvicola (^4. gulielmi) upon the lower jaws of the same form 

 from the same cave. The name gulielmi is thus available and 

 tenable for one of the British fossil species of Dicrostonyx. In 

 1872 Forsyth Major * found that the m^ and m- in a skull of 

 D. hudsonius from Labrador agreed with those of the Quedlinburg 

 skull in lacking the postero-internal vestigial angle found in each 

 of those teeth in D. torquatus and (according to Sanford's figure) 

 in the skull from Somersetshire too. Forsyth Major was thus 

 led to suggest that two .species of Dicrostonyx, one agreeing in 

 dentition with D. hudsonius, the other with D. torquatus, might 

 have inhabited Europe during the Pleistocene period. Between 

 1872 and 1910 a large number of occurrences of fossil remains of 

 Dicrostonyx in the European Pleistocene were recorded, but no 



^ Hensel, Zeitschr. deutsch. geol. Ges., 7, p. 492. 



- Ibid., 8, 1856, p. 279. 



' San-ford, Q.J.G.S., 26, 1870, p. 125, pi. viii, figs. 2 and 4. 



* Forsyth Major, Atti Soc. Ital. Sci. Nat. Milano, 15, p. 125. 



