xl MOLLUSCA OF SOMERSET. 
The Swan Mussel, Anodonta cygnea, Linné, is frequent in 
the majority of the larger ponds, canals, and rivers, where it 
prefers a muddy bottom. It is of gregarious habit, and fre- 
quently attains considerable dimensions. It is the largest of 
our freshwater bivalved molluscs, safe from foes (excepting a 
few internal parasites) in its deep-water home, it probably 
lives to a great age. The canals teem with molluscan life. 
In the open water we find : 
Anodonta cygnea, Linné; associated with 
Limnea peregra, Miiller. 
Valvata piscinalis, Miller. 
Pisidium amnicum, Miiller. 
Sphezrium corneum, Linné (local). 
Spherium lacustre, Miller. 
Neritina fluviatilis, Linné (on rocks). 
Dreissensia polymorpha, Pallas (local). 
In the reed-belt of the canals, rivers, and ponds, the associa- 
tion is the same as that of the reed-belt of the rhines on the 
Levels (see p. xxviii); the associations of the respective 
margins are also identical. 
Anodonta cygnea is absent from some of the rivers, and 
Neritina fluviatilis is to be found only on stones in slow- 
running waters. 
The shallow non-calcareous streams on the heathy sandstones, 
e.g. Penselwood, yield : 
Planorbis albus, Miiller. 
Ancylus fluviatilis, Miller. 
Limneza peregra, Miiller. 
Pisidium pusillum, Gmelin. 
The cattle ponds on the grass-lands yield Spherium lacustre, 
Miller, a highly specialised species with closely fitting valves, 
able to retain life for a long time in summer drought, buried 
in the moist clay of ponds that are quite dried up on the 
surface, 
