244 lOIiRNAI. OK CONCTTOI.Ol'.V, VOL. r6, NO. 7, SEI'TEMKKK, 1 92 1 . 



jS 4. Rivets and Streams. 



The River Nene, here of some width and good depth, affords a 

 great variety of the fresh-water species, most of them in great abun- 

 dance. We can separate it into three types of habitat, each with its 

 own association of niolluca ; («), Open Water, the deeper parts of 

 the river, where are the water-weeds growing from the river-bed ; {b), 

 Reed-Belt, where Reeds and water-plants fringe the banks; ((:■), Marsh, 

 the rush-grown shallows and moist river-margins. 



((?), the open water affords : — Velktia lacustris, Ancylus fluviatilis, 

 Lwutcea pereger, L. ixuricularia, Planorlns albus, Vivipara confeda, 

 Valvata piscinalis, Unto pictorum, U. tiimidiis, Anodcmta cygnea, 

 Pisidiuin amnicum, P. nitidurn, P. pulchelhim, P. stipinuvi^ and on 

 stones and stone-work of locks and bridges Neritina fluviatilis and 

 Dreissensia polymorpha . 



(Ji), the Reed-Belt affords : — Limncea palustris, L. stagfialis, Plan- 

 orbis Cornells, P. unibilicahts, P. spirorbis, P. vortex, P. fontaftus, P. 

 contortiis, Physa fontinalis, Bithynia te?itaculata, B. leachii, Valvata 

 cristata, Sphcerium cornemn, Pisidium milium, and P. subtriincatum. 



{c), the Marsh affords : — Agriolimax agrestis, A. Icevis, Hyalinia 

 radiatula, Arion ater, A. minimus, Vallonia pulchella, Siiccinea putris, 

 S. elegans, Catyclmim mininmin, Limncea truucatula, Pisidium henslow- 

 aJium, and P. casertanum. 



There are also two small rivers which touch the district ; swift, 

 shallow streams with deeper pools. They are by no means so rich in 

 mollusca. Open Water : — Limncea pereger, Valvata piscinalis, and 

 IJjiio pictomm. Reed-Belt : — Planorbis timbilicatus, P. spirorbis, 

 Sphcerium corneum, Pisidium nitidum, P. casei'tanitm, P. milium, P. 

 subtruncatufu, and P. hensloivanum ; and in one case Pisidium 

 hibernicum. 



The many small brooks contain as a rule an abundance oi Limncea 

 pereger, often without any other species. 



The association appears to be : — 



Dominant: Limncea pereger; with (fairly constantly) Spluzritini 

 corneum and Pisidium subtruncatum ; and (occasionally) Litnncea 

 truncatula and Planorbis spirorbis. 



C. — Woodlands. 



The natural woodlands of England may be roughly divided into 

 three great series : — 



I. — The Alder-Willow Series (on very dar > soils). 

 II. — The Oak and Birch Series (on siliceous soils). 

 III. — The Beech and Ash Series (on calcareous soils). 

 (To be concUtded). 



