148 JOURNAL OF CONCHOLOGY, VOL. IT, NO. 5, JANUARY, I905. 



end. These cliffs are entirely composed of marl with no other 

 admixture, except perhaps occasional long black stems of plants, 

 resembling Equiseta, and at one point a number of angular limestone 

 boulders are included. There is also a thin seam of remains of 

 mosses, about three inches thick, at a depth of four feet from the 

 surface. Thinking we should bottom the deposit, we dug down here 

 for over a foot, but were not successful. A sloping beach extends 

 from the foot of these chffs, and is strewn with immense numbers of 

 dead shells, bleached quite white, and composed for the most part of 

 LiimicEa pereger and BWiytiia tentaailata. The long series of white 

 cliffs and the shell-strewn beach make the shore a conspicuous object 

 when seen from the main road some distance away. 



The vast number of shells accumulated here is the result of the 

 water of the tarn being driven up against the small cliffs by prevalent 

 south-west winds, consequently large masses of deposit are under- 

 mined and eventually fall down. In course of time these masses are 

 disintegrated by the action of the water, and the shells freed from 

 the matrix. They are then deposited in various degrees of fineness 

 and weight, and occur in long rows fringing the shore, while the 

 vegetable debris washing up from the tarn, mingling with them, gives 

 the place the appearance of a sea beach. In some cases the mass 

 seems entirely made up of Limticea pereger, in others Bithynia tenta- 

 culata appears to predominate, and in one or two cases the opercula 

 of the last-named species occur to the entire exclusion of anything 

 else, except perhaps shells of Valvata piscinalis and V. ristata. Planorbis 

 crista occurs also, but is generally found under the water's edge. 



Although this mechanical sorting is useful in some respects, it does 

 not necessarily follow that the shells thus accumulated are derived 

 from the deposit itself, for mixed with them are many dead shells of 

 terrestrial origin {e.g., Carychium ininimum., Cochlicopa lubrica, 

 Hygromia hispida, etc.), together with dead aquatic species washed 

 up from the tarn. To be absolutely sure of the species actually 

 occurring in the deposit, we brought away large pieces of it to wash 

 and sort out at home, with the result that we found the following : — 



Bithynia tentaculata - Common. 



Limticza pereger - Common. 



Valvata piscinalis - Common. 



K cristata - - Not common. 



Planorbis crista - Rare. 



Pisidiiwi obtusale - Common. 



P. pusilluni - - Common. 



P. gassiesianuin - - Rare. 



All the shells of the Pisidia had both valves adherent ; the oper- 

 cula of Bithynia also occurred in various stages of growth. There 



