180 THE ZOOLOGIST. 
in the soft mud. The Unios appear not to have the power of 
doing this. At such times, too, a great number fall victims to 
the rats, which break their shells and eat the mollusks inside, 
though the thicker shells of the Unios defy them. 
UNIVALVES (GASTEROPODA). 
Order PEcTINIBRANCHIATA. 
Fam. NERITIDE. 
Neritina fluviatilis—Many in the Foss near Yearsley Lock. 
Very abundant in the Ouse both above and below the city, but 
particularly so just above Scarbro’ Railway Bridge, where there 
are a great many stones which they frequent. All those in the 
Ouse are more or less what Mr. Jeffreys refers to when he says, 
“Mr. North has found a black variety in the Ouse.” Whoever 
bestowed upon it the name of “ variety nigrescens” might well 
have saved himself the trouble, for in my idea its colour is due 
to nothing but the impurities in the water. 
Fam. PALupDINIDS. 
Paludina contecta.—“ Stream near York,” say Dixon & Watson, 
doubtless referring to the locality near Cherry Hill well known to 
Bootham conchologists of ten years ago, but now almost forgotten. 
In October, 1877, I visited the stream this mollusk used to inhabit, 
but found it now so surrounded with houses and in such an 
indescribable state of filth, from the refuse of some works being 
thrown into it, that it was difficult to imagine any Mollusca living 
there. I and others have visited it unsuccessfully, and I am 
afraid it must now be looked upon as a locality of the past, 
though I have more than once found dead specimens near by. 
Paludina vivipara.—This is difficult to obtain when the water 
in the Ouse is at its full height, but when it is lowered, as it was 
in August, 1876, I could obtain it in any quantity. Above the 
city it is rather uncommon, but below it abounds, and lives 
principally clustered round the mouths of the small ditches and 
drains that discharge into the river on the Bishopthorpe side. 
Probably it prefers these situations on account of the sewage and 
filth there to be obtained. Many of those obtained in 1876 con- 
tained young, forty or fifty in each being no uncommon number. 
I was much interested in watching some that I had in an aquarium, 
where, under the influence of clear water, in a few weeks they lost 
