Norwich Castle Museum. 121 
small but very abundant. The genus Planordis isa 
peculiar one, the shells of its members are coiled so 
as to form a flat or even concave surface like an 
Ammonite, and their inhabitants seem too small for 
their habitation. When irritated they emit a purple- 
coloured fluid, and if left dry, as not unfrequently 
happens (they often inhabit shallow water), they 
close the mouth of the shell with an epiphragm, 
and retiring into its recesses wait for better times. 
There are several species, and /. corneus, well 
known in this neighbourhood, may be regarded as 
typical of the genus. ‘The pretty delicate shells of 
Physa fontinalis should be noticed. The species is 
common and very interesting. It may frequently be 
seen floating at the surface of the water, foot upwards, 
and is said to spin a filament to lower itself to the 
bottom should no leaf or stalk be near. Lzmnea is 
another pond genus, containing some beautifully- 
formed shells, notably ZL. prlustris and L. stagnalis, 
the latter the largest of the family, whilst Z. auricularia 
is remarkable for its rounded form and expanded ear- 
shaped aperture. They are all inhabitants of marshes 
and ponds. Axcylus fluviatilis, known as the fresh- 
water Limpit, is a curious form widely distributed, dwell- 
ing in brooks and shallow waters, attaching itself to 
stones and shells. Though undoubtedly air-breathing, 
it is still believed to be capable of extracting the air 
in some degree from the water for the purpose of 
respiration, this, from the length of time it remains 
submerged, must be necessary. There is a second 
species known as A. /acustris, which attaches itself to 
the under sides of the leaves of water plants. 
Case Il. 
We now arrive at the Terrestrial Univalves. The 
