RIVER GARDENS, ETC. 
Snail alone has been found to fulfil these functions 
with great completeness, hut as variety of form is a 
great desideratum when it is sought to render an 
Aquarium an ornamental object, several other kinds 
of fresh-water mollusca may he added. The Trumpet 
Snail, with its flat coiled form, similar, in miniature, 
to that of the giant Ammonites , so remarkable among 
the shells of a former epoch of the world’s natural 
history, will, for instance, form a pleasing contrast 
to the sharp spiral of the shell of the Limneus 
pereger , or small Mud Snail. Then there is the 
beautiful though common Marsh Shell, Faludma 
vivipara , and its relative, 'Paludina achatina , both 
of which would he ornamental as well as useful. 
These species only attack the small decaying portions 
that fall from the plants, or the minute confer void 
growth that attaches itself to the glass. But some 
other species are very destructive to the plants 
themselves. Among these the larger Mud Shell, 
Lymneus stagnalis , is most voracious in an Aqua¬ 
rium, and a couple of them would soon clear it of 
its little forest of aquatic vegetation. Some of our 
bivalves of the fresh-water Mussel tribe may he 
added, for variety of form, though it is not yet 
ascertained whether they are useful as cleansers. 
Among these the Swan Mussel ( Anodon cygneus) is 
47 
