XXX MOLLUSCA OF SOMERSET. 
may be dragged out by the scoop. The smaller species of 
Planorbis, Physa fontinalis, and the young of Limnea peregra 
creep on the lower (submerged) surface in great numbers. 
In rhines containing but little weed and much “green 
scum” I found Bithynia tentaculata in great plenty associated 
with L. palustris, L. stagnalis, P. nitidus, and V. piscinalis. 
From a rhine covered only with a single species of Ranunculus, 
apparently typical R. heterophyllus, Weber, I dredged Limnea 
peregra and Bithynia tentaculata, neither in great numbers. 
In the largest rhines and in the canals the greater width of 
the channel lessens the competition between the plants, and 
the open centre permits sufficient light to enter the water to 
allow of the growth of submerged leaf plants such as Ranun- 
culus circinatus, Sibth; Hippuris vulgaris, Linné; various 
species of Potamogeton and Chara, Utricularia vulgaris, Linné ; 
Hottonia palustris, Linné; and Myriophyllum verticillatum, 
Linné. Here we find the following association : 
Limneza peregra, Miiller. 
Valvata piscinalis, Miller. 
Anodonta cygneza, Linné (local). 
Spherium rivicola, Leach (rare). 
Spheerium lacustre, Miiller. 
Pisidium amnicum, Miiller. 
(2). Peat-Moor Formation. 
The molluscan fauna of the rhines of the peat-moors much 
resembles that of other parts of the Levels. Succinea elegans 
is the dominant species in the reed belt, and often occurs in 
multitudes on the stone walls of the bridges crossing the lesser 
rhines. Two or three large slabs of Lias stone lying flat on 
the walls form the bridge proper; swallows not infrequently 
build their nests on the walls just below the slabs. 
Molluses are certainly less abundant (as may indeed be 
noted throughout the Levels), both in species and numbers, 
in rhines containing no duckweed. Wherever Lemna occurs 
