Pyrgulopsis Scalarifor.mis. 169 



the Pvrgiilopsis in the loose talus of sand 13'ing at the foot of 

 the bank. 



It wil be observed upon an examination of Fig. i, that the 

 current in this arm of Rock River impinges upon the bank 

 between the points a and b^ and that consequently this portion 

 of the bank is not now receiving, nor has it recently received, 

 additions, but on the contrary is being eroded by the current. 



This fact led the writer to believe that the numerous 

 bleached shells of the species in question which were found in 

 the talus [d Fig. 2) were not recently deposited b}^ the stream 

 but that they formed a portion of the broad alluvial deposit 

 which extends far to the north from the points mentioned. 



A careful examiination of the deposit during this and subse- 

 quent visits confirmed this belief. Sections were made at 

 numerous points between a and b, (Fig. i) along the face of 

 the bank which is here about seven feet above low w^ater, 

 and three borings ^ were made respectivel}^, 40, 50, and 150 

 feet from the edge of the bank in the vicinity of the point c. 

 Fig. I. 



All of these sections gave substantially the same results, 

 and the one figured (Fig. 2) which was made near the point 

 c, Fig. I, will serve as a type, and clearly shows the structure 

 of this alluvial deposit. It consists of three more or less dis- 

 tinctly defined strata. The uppermost stratum, [a. Fig. 2) 

 about three feet in thickness, consists of very fine, rich, black 

 alluvium in which are found a few shells of Alcsodon, Suc- 

 cinea, LlmncBa^ Physa^ etc., the species being almost exclu- 

 sively Piilinoiiata. 



The second stratum (/>• Fig. 2) about two feet in thickness, 

 contains an admixture of the materials of the first and third 

 strata, the relative proportions varying however. There is 

 also a corresponding mixture of the shells of the two strata. 



The third stratum, (c, Fig. 2) which was penetrated to a 

 depth of four feet, consists of pure sand and fine gravel, and 



I A seven-inch post-hole auger, attached to sections of gas-pipe, was used. 



