470 H. J. FLEURE AND MURIEL M. GETTJNGS. 



mainly through chemical sense impressions, which must be 

 a first step in the evolution of the mating tendency. 



Probably some of the ancestors of the Montocardia, which 

 have mating habits, went through somewhat similar stages 

 in the course of their evolution. Prof. Ainsworth Davis has 

 suggested to us that the egg-eating habit of certain species 

 of Trochns should also be considered in connection with the 

 development of mating habits. 



Our material was obtained from the shores of Cardigan 

 Bay, south of Aberystwyth, which show for many miles as a 

 plane of marine denudation formed by fairly evenly worn 

 ridges of Silurian grits and slaty shale. The old valleys in 

 the Silurian rocks, of which most of the coast cliffs are 

 built, are filled in some cases by the boulder clay of the 

 glacial epoch. The small streamlets which run down from 

 the cliffs thus drain sometimes off the boulder clay, and 

 sometimes off the shales and grits. We find that in several 

 cases T. crassus occurs in abundance where the shore is 

 washed by streams from the boulder clay, but disappears 

 sometimes almost abruptly when the boulder clay makes way 

 for the Silurian rock. The same thing is true, though in a 

 lesser degree, for Trochus obliquatus, and the fact seems 

 to be of sufficient interest to be chronicled, though we have 

 not yet been able to approach an explanation. The boulder 

 clay is found on analysis to be rich in salts of potash and 

 magnesium. Trochus is not found much within three miles 

 of Aberystwyth at present ; the boulder clay in this imme- 

 diate neighbourhood has probably been carried by a different 

 set of glaciers from that flowing seawards further south. 

 The pollution of the Ystwyth and Rheidol rivers by lead 

 works may also be a determining factor. 



List op Papers quoted. 



1. Ainsworth Davis, J. R. — " Bionomical Considerations in Gastropod 



Evolution," 'Journal of Malacology,' 1905. 



2. Drummond, Miss. — "On the Development of Paludina," 'Quart. Journ. 



Micr. Sci.,' vol. xlvi, 1902. 



