RIVER GARDENS, ETC. 



Snail alone has been found to fulfil these functions 

 with great completeness, but 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 be 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, Paludina 

 vivipara, and its relative, Paludina achatina, both 

 of which would be ornamental as well as useful. 

 These species only attack the small decaying portions 

 that fall from the plants, or the minute confervoid 

 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 be 

 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 



