ADDITIONAL NOTE ON HELICINA OCCULTA 233 



species, as well as the example of other terrestrial shells such as 

 Succinea obliqua, Polygyra multilineata, etc., would suggest rather 

 that local variations in the minimum amount of moisture are respon- 

 sible for the differences in size, the smallest shells occurring in the 

 driest or most exposed places. The average diameter of both recent 

 and fossil forms becomes a little less as we go westward into drier 

 regions, and "depauperation," so far as it exists not only in this, but 

 in other species which occur in the loess, points decidedly toward a 

 dry rather than a cold climate. 



Perhaps further emphasis should be placed upon the distribution 

 of both the recent and fossil forms of this species. Reference to the 

 first paper on Helicina occulta 1 shows that the northernmost points 

 at which recent specimens of that species were collected are Winona 

 and Stockton, Minn., and De Pere, Wis.; the westernmost point is 

 Eldora, Iowa; the most southerly point is South Pittsburg, near the 

 southern line of Tennessee; while eastward the species extends 

 beyond the mountain to Virginia. The southern limits of distribu- 

 tion He beyond the border-line of the southernmost drift-sheet, the 

 Kansan, and the eastern limits extend beyond the southeastern 

 border-line of the Wisconsin drift. Northward the species extends 

 locally over the Iowan, and a portion of the Wisconsin drift areas 

 (in Minnesota ( ?) and Wisconsin), and also over a part of the drift- 

 less area in Iowa. 



The fossils are more restricted in both their northerly and southerly 

 distribution, but extend farther west, being known from the south- 

 western part of Howard County, Nebraska. 2 The known northern 

 limit extends from Cuming County, Neb., to Woodbury and Johnson 

 Counties, Iowa, Moline, 111., and Sullivan County, Ind. 3 The 

 southern limit, so far as is now known definitely, is at Kansas City, 

 Booneville, and St. Louis, Mo.; Gallatin County, 111.; 4 and Posey 

 County, Ind. Owen also mentions Helicina, without specific name, 



1 Loc. tit., pp. 174, 175. 



2 See American Geologist, Vol. VII (1891), p. 40. 



3 Reported from the last locality by Collett, Second Report of the Geological Sur- 

 vey of Indiana (1871), p. 227. 



4 Reported by Cox, Geological Survey of Illinois, Vol. VI (1875), p. 210; Eco- 

 nomic Geology of Illinois, Vol. Ill (1882), p. 561. 



