470 THE ZOOLOGIST. 
NOTES ON AQUATIC MOLLUSCA OCCURRING IN THE 
NEIGHBOURHOOD OF PONTEFRACT. 
By GrorcE Roserts. 
(Continued from p. 429). 
Tue principal places for aquatic Mollusca in the Pontefract 
district are the ponds and ditches near Castleford and Fairburn 
on the north side, and the small River Went on the south side, 
together with the canals at Castleford and Knottingly. The 
canals, however, are very much less prolific in shells than they 
were five-and-twenty years ago, owing mainly to the pollution of 
the water. Near Castleford there are a good many ponds and 
ditches filled with water-weeds, and in these a considerable 
number of species and varieties occur, but great numbers are 
destroyed every year by cleaning out the ditches. Wherever the 
Canadian weed, Anacharis Alsinastrum, occurs, shells are to be 
found, and it is perhaps owing to the extension of this alien 
plant (introduced about 1841) that certain species, such as 
Planorbis contortus and Limnea stagnalis, have become more 
frequent. 
In the Rivers Aire and Calder, which are now simply 
great open sewers, we find absolutely nothing. Many shells 
may be found in the tributary streams, but in these they seem 
to be of very uncertain occurrence, a consequence probably of 
displacements and destruction, caused by floods; and, in the 
canals, very many are destroyed by the vessels continually 
passing to and fro, by dredging, and by clearing out the water- 
plants from the sides. Eastward of Pontefract the streams are 
slow-running, and the average height above sea-level may be 
about 100 feet. 
II—AQUATIC MOLLUSCA. 
LAMELLIBRANCHIATA. 
Fam. SpH@rupm. 
Spherium corneum, Linn.— Castleford, Methley, Ackworth, 
and other places ; common. 
Var. flavescens, Macgill.—Castleford and Fairburn. Some 
are much more shining than others. 
S. rivicola, Leach.—River Went. 
