472 THE ZOOLOGIST. 
Fam. DREISSENIDE. 
Dreissena polymorpha, Pallas. Zebra Mussel. — Abundant 
some years since at Ferrybridge, where in some places the sides 
of the canal were lined from top to bottom. ‘‘ Often eroded ; 
but in some stations, very limited in extent and apparently 
nowise different from the contiguous parts where the eroded 
specimens occur, they are not eroded.”—J. Wilcock, in MS. 
PECTINIBRANCHIATA. 
Fam. NERITIDZ. 
Neritina fluviatilis, Linn. River Neritine.—On stones in the 
shallow parts of the River Went. Formerly near Castleford. 
Black, yellow, brown, and variously spotted and streaked 
varieties occur. 
Fam. PaLupinip”. 
Paludina vivipara, Linn. Common Marsh Shell.—Brotherton ; 
small in size. Often much decollated. 
Bythinia tentaculata, Linn. Tentacled Bythinia.—Common in 
nearly every stream. 
Var. ventricosa.— Abundant in the River Went; in some 
places in greater profusion than the type. Specimens of this 
variety, occurring in the Went, vary in colour from horn-colour 
to white, and also in degree of thickness, some being semi- 
transparent ; the latter are found at Smeaton, near Pontefract, 
but not common.”—J. Wilcock, in MS. 
Var. excavata. —Common. As this variety is only dis- 
tinguished by having a rather deeper suture, it should be merged 
with the type. Nearly all univalves vary in the depth of the 
suture. 
Var. alba.—River Went, and near Askern; small. 
Monst. decollata.—Castleford and other places ; sparingly. 
B. Leachii, Shepp. — Knottingly Canal, amongst the bur- 
reeds, and at Norton, near Askern. Specimens with a deeper 
suture than the recognised type, and more produced spire, are 
often found. 
Fam. VatvaTip&. 
Valvata piscinalis, Mull. Stream Valve Shell.—Near Castle- 
ford, and in the River Went. Monstrosities are various; some 
have the apex or one or two whorls intorted. 
