_Pisidium amnicum._--(Our five _pisidia_ resemble _Sphærium_, but are much smaller, all but _amnicus_ being minute. Very abundant where found. _P. amnicum_ and _fortinale_ are triangular in shape, _P. pusillum_ oval, _P. nitidum_ round, and _P. roseum_ or _milium_ oblong; but they are difficult to distinguish on account of their similarity and variation). _P. amnicum_ is nearly twice the size of the others, and this and _fontinale_ may be found in slow rivers, whereas the others prefer stagnant waters.
_Pisidium fontinale._--Smaller and thinner, and with more prominent beaks than _P. amnicum_.
_Pisidium pusillum._--The most common species. Distinguished from the last by being oval and by its beaks being blunter and more central.
_Pisidium nitidum._--Rare. Very glossy and striated.
_Pisidium roseum_ (from the colour of part of its body).--Like _nitidum_, but oblong, with a straight lower margin, and with beaks placed away from the centre.
The last shell to be mentioned could not be mistaken for any other. It belongs to the sub-order _Heteromya_ (_i.e._ with adductor or closing muscles not equal); to the family of _Mytilidæ_ (or mussels) and the genus _Dreissensia_ (named after a Dutch conchologist).
_Dreissensia polymorpha_ is a triangular, boat-shaped, bivalve, supposed to have been introduced with Russian timber (as was also probably _Hydrobia Jenkinsi_). It is gregarious, and attaches itself to objects by a byssus like our marine mussels. Shell yellowish-brown with wavy purplish lines, wrinkled in the line of growth. Common in the New River, and has been found in iron water-pipes in Oxford Street.
All our shells have varieties (many an albino or white form), and the collection and distinguishing of these varieties, which in some species are numerous, adds much to the interest of the collector. In addition there are also the variations in size or markings which can hardly rank as varieties. Inasmuch as none of our shells are peculiar to our country (which is from the natural history and the geological point of view only a detached portion of the Continent), it may be well to warn young collectors that if they receive shells from the Continent, mere varieties are there often named as separate species and variations considered as definite varieties. This is especially the case with _Helicella virgata_.