122 CHAS. L. WALTON. 



that there is a correlation between such distribution and shore drainage. 

 The barren area receives most of its shore drainage from lime-free slates 

 and grits, while the " Trochus "' areas receive water from boulder clay in 

 the southern, and the same, and igneous rocks, in the northern portions 

 of the Bay. The streamlets followed by M. crassa drain from boulder 

 clay. An attempt was made to obtain additional data by a comparison 

 of the Algce from various portions of the Aberystwyth district. Certain 

 differences were discovered (see list of Alga'), but none of them appear to 

 have any direct connection with the Mollusca under consideration ; and 

 the evidence afforded is thus more cumulative than decisive. The 

 boulder clays of different districts appear to have different lime con- 

 tents. The water of the New Quay district is " hard,"' fruit trees 

 grow well there and the drifts probably contain lime, whereas the 

 boulder clay from the region drained by the Ystwyth has been analysed 

 by Dr. T. C. James, and found to be quite devoid of lime. The northerly 

 drift along the coast has been mentioned already ; and it is noteworthy 

 that drainage from the areas containing lime would tend to influence the 

 coast some distance northward on that account. The limeless drainage 

 of the Ystwyth, moreover, coincides with an area of slate and shale rocks ; 

 and this is the most barren as regards Trochidse. The reappearance of the 

 Trochidse to the north again coincides with the reappearance of boulder 

 clay, and river drainage from districts containing volcanic rock, etc. 



BIBLIOGRAPHY. 



1853. Forbes, E., and Hanley, S. History of BrititsJi Mollusca. London, 



Van Voorst. 

 1886-92. Reports upon the Fauna of Liverpool Bay and the Neighbouring 



Seas, L.M.B.C. 

 1887. Tryon, G. W. 3Ianual of Condi ologij, Vol. IX, p. 229, etc. 

 1896. The Marine Zoology, Botany, and Geology of the Irish Sea ; Fourth 



and Final Report of the Committee. 

 1904. "Plymouth Marine Invertebrate Fauna," Journ. Marine Biological 



Assoc., Vol. VII, No. 2. 

 1907. Fleure, H. J., and Gettings, M. M. "Notes on Common Species 



of Trochus," Q.J.M.S., 1907, pp. 459-472. 

 1911. Walton, C. L., Fleure, H. J., and Wright, F. S. National Union 



of Teachers Souvenir Volume : Aberystwyth Conference. 



" Notes on the Marine Life of the Shores of Cardiganshire," 



pp. 97-110. 



