DICROSTONYX 147 



has been used by G. M. Allen ^ recently to divide the genus into 

 two subgenera, viz., Dicrostonyx containing the only hving species 

 that lacks the vestigial angles, and Misothermus comprising all 

 the others which retain, or, as Allen thinks, have acqiiired them. 

 In the mandible, mj consists of a posterior loop, seven closed 

 triangles (of which four are internal, three external), and an 

 anterior loop compounded out of at least four more or less reduced 

 and confluent triangles (Fig. 72, i-s, lo) ; occasionally the 

 postero-external triangle, normally blended in the anterior loop, 

 is shut off as an eighth closed triangle ; this tooth has never less 

 than nine dentinal spaces, and five outer and six inner salient 

 angles ; but occasionally one of the usually ephemeral elements 

 of the anterior loop maintains its independence and persists, 

 forming an additional outer or inner salient angle (Fig. 72, 2) ; 

 >/?2 has a posterior loop, four alternating triangles, and a pair of 

 vestigial angles in front ; of the latter the outer vestige is the more 

 reduced, and the dentine of the vestigial pair is confluent with 

 that of the fourth triangle ; not counting the vestiges this tooth 

 has three well-developed salient angles on each side; m^ is like 

 Wj, but a little more reduced ; its third or antero-internal triangle 

 is sometimes partially confluent with the fourth ; of the vestigial 

 angles in front, the outer one is lost in many species, while in 

 D. hudsonius both the inner and the outer vestiges are lacking. 



Owing to the want of material from the Old World it is 

 impossible to say how many species of Dicrostonyx exist and to 

 determine what status should be accorded to several of the forms 

 currently recognized. We are indebted to G. M. Allen for a 

 revision of the American members of the genus, and free use 

 has been made of his work in preparing the accounts of the 

 described forms given below. 



Allen arranges the species, as indicated above, in two sub- 

 genera, Dicrostonyx and Misothermus, distinguished by the 

 presence or absence of the postero-internal vestigial angles in 

 m} and nfi. But this difference, although useful for discriminating 

 between species and very constant in American and in European 

 fossil species, is to my mind altogether too slight a foundation 

 for subgenera, particxilarly in view of the fact that in the Old 

 World D. torquatus the characters of m^ and trfi appear to be 

 rather inconstant (Figs. 71, 1 and 71, 2). Whether we regard 

 the minute postero-internal angle so frequently present in these 

 two teeth as an ancient vestigial structure (my view) or as a 

 rudiment of a new complication (Allen's view), it seems to be just 

 such a character as may from time to time vanish or appear 

 quite independently in any member of the genus; no con- 

 clusion as to the special interrelationships of the known forms, 

 fossil or recent, can be based upon the mere presence or absence 

 of such a structure. 



1 G. M. Allen, Bull. Mus. Comp. Zool. Harvard Coll., Cambridge, 

 Mass., 62, p. 513. 



