COMPARISON OF AFTONIAN WITH MODERN MOLLUSKS 123 



avara and one each of Vitrea and Yallonia gracilicosta were secured in 

 this way, together with one specimen of Yallonia parvula Sterki, a species 

 not known from the Aftonian. 



It is important to note the small number of terrestrial shells which 

 find their way to the bottom of the bay, especially since terrestrial species 

 are abundant in the groves bordering the bay, and great numbers of them 

 are carried out on the lake by freshets to be thrown ashore again. This 

 was well illustrated during the past summer, Avhen great numbers of 

 shells, largely terrestrial species, were thrown out on the north shore of 

 Miller Bay. This bay is bordered by native forest on the west and south 

 shores, and in these groves dwell more than 25 species of terrestrial mol- 

 lusks, some of them in large numbers. The dead shells of these species, 

 often perfectly fresh, are washed into the bay, and are driven to the north 

 shore by the prevailing southerly or southwesterly winds, where they are 

 literally massed in thousands on the flat beach,^ together with aquatic 

 species from the bay itself. Yet notwithstanding the great number and 

 variety of forms® which are so carried across the ba}^, but few find their 

 way to the bottom, as was shown by dredging. 



EVIDENCE oF CLIMATIC AXD SURFACE CONDITIONS 



If now we apply approximately the same ratio to the terrestrial mol- 

 hisks which are found in the Aftonian, we will find the tei;^estrial species 

 even more abundant, and the climatic and surface conditions which made 

 their existence possible much more general and widespread than might 

 appear possible from the study of the extent of the Aftonian deposits 

 themselves ; for, with the exception of these few land shells, the evidence 

 furnished by the Aftonian points to great floods, as the deposit was evi- 

 dently largely formed by strong currents ; yet the presence of these land 

 shells (together with the presence of large terrestrial mammals) is suffi- 

 cient to show that extensive land areas did exist. 



Importance of exact Determination of Aftonian Horizon 



In view of the foregoing considerations, it is important that, so far as 

 possible, all doubt concerning the stratigraphic position of the beds which 



8 No forest suitable as a habitat for these species occurs on the slopes adjacent to the 

 north shore of the bay, and the only possible source of these shells Is on the opposite, 

 or southerly, side of the bay. 



9 In the drift material on the north shore of the bay the writer found the following 

 aquatic forms : 1 species of Pisidiiim, 1 Amnicola, 1 Valrata, 5 Planortis, 1 Segmentina, 

 4 Lymyxcea, and 1 Bulinus. Mingled with them in greater numbers were the following 

 terrestrial forms : 1 species of Carychium, 1 Euconnlus, 2 Zonitoides, 1 Vitrea, 1 Pyra- 

 miditla. 1 Helhoflisciis, 1 Punctvm, 1 Strohilops. 5 Bifldaria, 3 Vctigo, 1 Pupoides, and 

 1 Cochlicopa. Hundreds of specimens of several of these species /ere collected. 



