Of the genus Pisidium^ we have two or threa 
species in the district, P. fontinale, P. nitidum — 
one or more of which are found in almost every pond. 
They are generally amongst mud, and being small 
and very often coated with a browm coloured sub¬ 
stance, they are apt to be passed over. I was sur¬ 
prised a few days ago to find one of these shells in a 
small drain near the top of Murray shall Hill. It 
is the general opinion of naturalists that water 
mollusca are distributed by becoming attached to 
the feet of waterfowls, and I suppose that may 
account for this little creature being found so high up. 
The food of this, as well as of the last-named genus, is 
animalculse. These are all the Bivalves found in the 
ponds around Perth. The other two orders, Unionidoe, 
being only found in rivers, and the Dreissena, only 
occurring to the south of Edinburgh in Britain. 
We now come to the second class of Univalves, 
which is di^dded into two orders. The first of these 
orders is again divi.ded into three British families, only 
one of which, the Valvatidoe, is represented here by one 
species Valvata jpiscinalis. It is found in MoncriefFe 
Pond and several other places along the sides of the 
rivers Tay and Earn, and throughout Britain. It has 
a very pretty little spiral shell of 6 whorls, and has an 
operculum very beautifully formed of 10 to 12 whorls. 
There is another species— Valvata cristata — said to 
be found all over Britain, and likely to turn up in the 
district although not yet observed. It has a flatter 
shell and .formed of only 5 whorls. Both of these 
species have a branchial plume, bearing fourteen 
slender ofifsets, which are placed at right angles to 
the stalk. They are, however, very shy mollusks, 
and it is not easy to get a sight of the plume. 
In the second order — Pulmonobra.nchiata — (so. 
named from the lung-like form of the gill) we have 
only the family LymnoedcE containing four genera. 
The first of these is Planorhis, which, as the name 
denotes, has a flat coil shaped- shell. This is the 
prettiest genus of all the fresh water mollusca — the.r 
shells being delicately and beautifully formed. They 
have the peculiai'ity of emitting a purple-coloured 
fluid, apparently as a means of defence against .their. 
