INTRODUCTION. XX1x 
Spherium corneum, Linné. 
Pisidium pusillum, Gmelin (=P. fontinale, Draparnaud),. 
Beyond the Upright-leaf association or Reed belt we 
frequently find the surface of the rhine covered with 
plants characterised by their floating leaves, amongst the 
dominant species we may mention Hydrocharis morsus-rane, 
Linné; Lemna minor, Linné (other species of Lemna occur, 
L. trisulea, Linné, abundantly in some parts), Glyceria fluitans, 
Br., and Ranunculus heterophyllus, Weber. Dr. Moss observes 
that the Upright-leaf and the Floating-leaf associations are 
kept in their respective positions by the ditching operations of 
the farmers, “ but for this work, the upright-leaf forms would 
eventually occupy the whole rhine, which would become filled 
with humus and silt. This process can be seen taking place 
in the disused brick-ponds which are not cleaned by the 
ditchers.”’ 
In rhines partially silted up we find Spherium corneum and 
Pisidium pusillum in the mud, the last mollusc to retain its 
hold under the gradual change of conditions is Limnea 
truncatula. 'The molluscs to be found in connection with the 
Floating-leaf association are : 
Limnea peregra, Miiller. 
Limnea palustris, Miller. 
Limneza stagnalis, Linné. 
Planorbis corneus, Linné (local). 
Planorbis umbilicatus Miller (dominant). 
Planorbis vortex, Linné. 
Planorbis spirorbis, Linné. 
Planorbis contortus, Linné. 
Physa fontinalis, Linné. 
Bithynia tentaculata, Linné. 
Spherium corneum, Linné. 
Limnea palustris and Limnea stagnalis are typical shallow- 
water species. The Lemna often forms such a dense carpet 
of vegetation on the rhine surface, that a piece a yard square 
