122 AMERICAN SPECIES OP VERTIGO. 



strongly thickened outer margin and the somewhat stronger 

 dentition" (Marts.). 



The locality is at or near Medora or Little Missouri station 

 of the Northern Pacific R. R., in Billings county, North 

 Dakota. It occurred with Euconulus fulvus, Vallonia pul- 

 chella- and gracilicosta, Pupilla muscorum and blandi, Gastro- 

 copta armifera, "P. pentodon var." ( i l = G. holzingeri), and 

 Succinea lineata W. G. B. V. &. arthuri is not known to 

 American conchologists, but the description suggests V. colora- 

 densis basidens. 



Dr. V. Sterki has sent a note on a Vertigo collected by Mr. 

 A. W. Hanham at Winnipeg, Manitoba, which I suspect to be 

 identical with arthuri. "Like coloradensis, though one speci- 

 men has somewhat of a rounded crest over the palate. There 

 is a strong, white callus in the palate, thickest where the 

 short upper palatal plica merges into it, and thinner at the 

 lower palatal. It does not extend up to the suture or to the 

 base. 



"Length 1.6, diam. 0.9 mm. 



"Length 1.5, diam. 0.8 mm." 



Group of Vertigo modest a. 



Moderately large species, 2 to 3 mm. long, without sharp 

 striation (except in V. m. iusculpta), and never having a basal 

 tooth, the tooth formula varying from 0-0-0 to 2-1-2. 



It is a circumpolar Arctic and alpine group, comprising 

 many races with few teeth or none. The collections at hand 

 are deficient in Siberian and Greenland Vertigines, and I 

 am therefore leaving as species various forms which may 

 prove to be merely local races of V. modest a, such as V. hoppii, 

 V. krausseana and V. arctica. The arctic and subarctic forms 

 have teeth as follows: 



Angular. Parietal. Cohimellar. Palatal. 



V. krausseana .... 1 or trace. 



V. arctica 1 1 Oorl 



V. a. extima 



V. TO. ultima 



V. hoppii 1 or 1 or 1 or 



V. modesta . 1 or 1 1 2 



