48 SCIENTIFIC SURVEY OF NORWICH AND DISTRICT 
varius and P. opercularis are abundant. Macoma balihica is common to 
these habitats and the estuaries but does not attain the robust forms of 
more sheltered stations. Other estuarine Lamellibranchs include Scrobi- 
cularia plana (shells of which have been dug from the alluvium within a 
few miles of Norwich), and Mya truncata. The mussel (Mytilus edulis) 
and the cockle (Cardium edule) are cultivated on the north coast in specially 
chosen beds known as ‘ lays’; the immature molluscs being collected 
in the Wash. The rearing of these shellfish is of considerable economic 
importance. Whelks (Buccinum undatum) are obtained in baited basket- 
like traps on the weedy grounds between Cromer and Hunstanton ; 
winkles (Littorina littorea) are collected in the Wash and to a lesser extent 
on the ‘ winkle stones’ on the chalky foreshore. Mud-littoral species 
are Littorina saxatilis which shows considerable variation on this coast, 
Peringia ulve which teems on Breydon (the typical form), and in the Wash 
region (mainly var. subumbilicata), Phytia myosotis, and Assiminea grayana. 
Freshwater Molluscs——Paludestrina Fenkinsiu is widely distributed in 
the east, and is the only species living in estuarine and fresh water ; it 
has been found in Breydon and as far up the Yare as Norwich, and in 
the Bure at Coltishall. The extensive system of dykes is very favourable 
to freshwater molluscs, though the Broads themselves for the most part 
produce few species. The stagnant waters abound in Limnea and 
Planorbis spp., which show interesting modifications as they come within 
reach of estuarine influence, e.g. between Acle and Yarmouth, Neritina 
fluviatilus occurs in Horsey Mere which is somewhat saline, but otherwise 
lives only in the upper reaches of our rivers. Of the freshwater Mussels, 
Anodonta cygnea is small in the rivers, thin shelled (but not eroded) in 
the shallower broads and reaches its greatest size in ponds ; Unio pictorum 
accompanies it where there is flowing water. Extensive deposits of 
recent freshwater shells are present between Surlingham and Rockland 
in channels connecting some of the smaller broads. 
Land Molluscs—The mossy hummocks in sallow carrs of the river 
valleys yield many small terrestrial molluscs including Vertigo substriata. 
Arianta arbustorum, and Fruticicola striolata abound in nettle beds near 
the rivers. The dunes are colonised primarily by Pupilla muscorum, 
Vitrina pellucida, and Candidula caperata which can live under conditions 
too stringent for other species; they are followed by Cepea nemoralis, 
Cernuella virgata, Xerophila itala, Theba cantiana and others. C. nemo- 
ralis is also widespread in hedgerows and sometimes attains a large size in 
fenny localities ; C. hortensis is rather local in this county. Pomatias 
elegans is confined to chalky districts and has been found near Hunstanton 
and inland at Whitlingham. 
Among the slugs, Avion ater keeps mainly to the river valleys ; Agrio- 
limax agrestis is ubiquitous occurring on the marshes, on dunes, as well as 
throughout the agricultural districts. 
Polyzoa—The marine species have received scant attention. _Pedicel- 
lina cernua and Bowerbankia have occurred on Tubularia colonies at 
Yarmouth haven. Flustra foliacea is perhaps the most abundant species 
washed ashore. The brackish ditches near Yarmouth contain Membrani- 
pora monostachys var. fusaria encrusting various debris. 
