236 B. SHIMEK 



tially the same as those of a number of terrestrial species now inhabit- 

 ing the same area, and also often associated with this species in the 

 loess. The present writer, long before he had access to Dr. Binney's 

 work, repeatedly called attention 1 to the fact that the climate during 

 the deposition of loess, as indicated by the fossils, was similar to that 

 which prevails in the same region at the present time, and H. occulta, 

 in common with a number of other species of terrestrial mollusks, 

 offers strong testimony to this effect. The species does not extend 

 far to the north, either in recent or fossil form, nor was it pushed far 

 south by the ice-sheets following the Kansan. We have direct evi- 

 dence of its existence only since the Kansan, but even if it occurred 

 previous to the advance of Kansan ice, as it probably did, it still had 

 plenty of room southward in which to persist. From this territory 

 it again spread northward, but was checked by each of the ice-sheets 

 following the Kansan. How far north the species extended during 

 the several intervals preceding the final advance of the Wisconsin 

 ice cannot be determined, as each advancing ice-sheet destroyed all 

 the evidence of their presence, if any existed. That its advance 2 to 

 the more northerly localities in which it lives today was com- 

 paratively recent, however, is supported by the fact that it has 

 nowhere been found fossil in these northern localities. Thus, while 

 recent specimens are not uncommon in Winneshiek and Howard 

 Counties, Iowa, the loess of these counties has as yet yielded no fossils 

 of this species. 



Helicina occulta is not arctic, nor is it associated in the loess with 

 species which are distinctively arctic. The fact that its range was 

 not greatly extended during the oscillations of the successive glacial 

 advances, and that it remained practically unchanged through all 

 the interglacial loess-forming intervals, indicates a greater lack of 



1 First in the Proceedings of the Iowa Academy of Sciences for 1895, Vol. Ill, p. 85, 

 and subsequently in several papers. 



2 The question might here be raised as to the manner in which such advance could 

 be made. While the writer does not contemplate here the discussion of this subject, 

 he desires to suggest that, among possible agencies, violent storms may be included. 

 The American Journal of Conchology, Vol. V (1870), p. 118, contains the statement 

 that "John Ford exhibited specimens of Gemma gemma Totten, remarkable for having 

 fallen during a storm which occurred at Chester, Pa., on the afternoon of June 6, 1869." 

 Gemma gemma is a small marine bivalve. 



