356 



Geographical Distribution of Mollusca. 



being a species of later date. Of those in the reed-belt L. 

 pahistris is recent and does not occur in the marl ; Bithynia 

 tentacidata is abundant. li\'ing and in the fossil state ; Planorbis 

 contort us sparingty fossil and recent. 



The following shows the probable grouping of the marl 

 species. 



Reed Belt. 



Open Water 



L. peregev ^liill. 

 Planorbis albus L. 



crista I^. 

 Valvata piscinalis JNIiill. 



Marsh. 



Pisidinni pusillium 



Gmelin 

 obtusale Pf. 



B. tentacidata L. [foot. 

 Planorbis fontanus'Light- 



contort us L. 

 Valvata cristata Miill. 

 Sphcsrium corneum I^. 

 Pisidiuni lutidum Jen. 



)}iilium Held. 



There are evidently no river deposits as Ancylus fiuviatilis 

 and Neritina fiuviatilis are absent, and the conditions of life 

 when the deposits were laid down would seem to be similar 

 to those obtaining to-day. viz., a deep lake fed by springs, 

 with a shelving shore fringed with reed beds. 



Hale ]\loss is a small patch of a few acres, mostly white with 

 marl and litter, covered by shallow peat, occupying what was 

 the deepest (30 feet) hollow of the ancient Burton Lake. The 

 proportion of shells in the marl, here is very much less than in 

 that at Hawses Water. The following species occur in the marl, 

 and may be grouped thus : — 



Open Water. 



L. persger ~Sl\\\\. 

 Vcdvata piscinalis ^liill. 



Reed Belt. 



Valvata cristata Miill. 

 Pisidium nitidum Jen. 



cinereum Alder, 



Marsh. 



L. truncatida Miill. 

 Pisid. pusillum Gmelin 



obtusale Pf. 



The noticeable fact here is the absence of the Planorbis 

 group and shallow w^ater species — pointing to the existence of 

 a well-filled lake of great dimensions, which has now entirely 

 disappeared. 



In the marl deposit, L. pereger far exceeds in number all 

 the other species. The shells are not lacustrine in form, but 

 are very similar to those of the Irish marls. In regard to this 

 deposit, Mr. A. S. Kennarcl says : ' Judging from your list, the 

 deposit is not a shallow water one, and was laid down in tw^o 

 to three fathoms Planorbis is a shallow^ water form, 



and not likely to occur in a deep water deposit.' 



' Bithynia tentacidata is rather a shallow water form, whilst 

 the hall-mark of shallow water is Limncea stagnalis and Limncea 

 palustris.' 



As will be seen, Plajiorbis and Bithynia are absent, only 

 deep water species being found. 



Naturalist, 



