VI PREFACE. 



to exhibit twelve distinct and permanent varieties of 

 Anodon Cygneus, and there can be little doubt but 

 more may yet be ascertained, by a careful comparison of 

 specimens from numerous waters. 



Almost every situation in the British Islands has its 

 Land and Fresh Water Shells. Lakes, Rivers, Canals, 

 Ponds, and Ditches abound with the Lymnaedae, Valva- 

 tidse, Cycladffi, and Unionidse; some of which lurk in 

 the mud at the bottom, or crawl upon aquatic plants; 

 while those of the Land localise in extremely varied 

 situations ; the Helicidse and Limacidse frequent the 

 bottoms of hedge-rows, gardens, woods, and meadows, 

 under stones, amongst nettles and other plants, while 

 the Pupidse, Clausilidse, and their congeners inhabit 

 mossy banks, old walls, the bark of decayed trees, and 

 other dry places. 



I have rejected all the species not indigenous to the 

 British Islands, with the exception of Dreissina poly- 

 niorpha, which, although an alien, has become a nume- 

 rous and prolific inhabitant of most of our Canals. 



In the choice of specific names, I have for the most 

 part adopted those of Colonel Montagu, whose work 

 is entitled to the highest praise, from the clearness and 

 simplicity of its descriptions of species ; many of which 

 were given prior to those of Continental writers on this 

 branch of science ; whose specific names I have given 

 in the numerous synonyms. 



I have added to the illustrations, figures of an animal 

 of nearly each genus, with short descriptions of their 

 Generic Characters in an appendix, which I thought 

 preferable to mixing them up in the text. 



