178 THE ZOOLOGIST. 
Var. Brochoniana.—* Used to occur at Layerthorp on the 
Foss, but I have not seen it of late years” (H.) In August, 1876, 
the Ouse was lowered, and the water remained for several days 
about ten feet below its usual level, affording an excellent oppor- 
tunity for shell collectors. Among other things seven or eight 
specimens of this variety were obtained under large stones. It is 
much larger than the type-form, very flattened, and the umbones 
small but prominent. Mr. Hey writes:—‘‘The profusion and 
variety of Spheriums in the Foss is remarkable, the whole genus 
abounding at one place; but it is grievous to have to mourn its 
growing rarity, as it is most unpropitious for mollusks when the 
water is let out of the river in winter and the poor wretches are 
frozen to death.” 
Pisidium amnicum.—Fairly common both in the Foss and in 
the Ouse, but I have not met with it elsewhere. 
Pisidiuwm fontinale-—Common. 
Var. Henslowana.—‘ Common in sand dredged from the river 
Ouse” (B.) } 
Var. pulchella.—Common enough, I believe. ” 
Var. cinerea.—I have met with this in the ponds on Tillmire, 
and a few other places. 
Pisidiwm pusillum.—Generally distributed (H.) Specimens 
from York in the British Museum. Hobmoor, Askham, ec. 
I once found myriads in a tiny ditch in the centre of a field 
between Bishopthorpe and York. The water was very strongly 
impregnated with iron and their shells encrusted with it. 
Var. obtusale.—Also specimens in the British Museum, Com- 
mon generally. 
Pisidium nitidum.—Streams at Hobmoor, according to Mr. 
Joseph Taylor, of Sunderland. 
Fam. UNIoNID&. 
Unio tumidus.—-In the Ouse, but very rare. Abundant in the 
Foss. Mr. Hey says that it was formerly very common in the 
latter river, but is now very rare on account of its being killed 
by the frost when the water is let out in winter. ‘“ There are 
two distinct forms:—(a) Wedge-shaped, veutricose, thick, and 
of a rich red-brown colour; (b) Longer, thinner, greenish, and 
almost the var. radiata. I have some specimens which are 
curved like margaritifer; others which seem intermediate between 
