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list, 123 are found here, though this may be partly accounted 

 for by remembering that with the exception perhaps of some 

 parts of Yorkshire and Lancashire, no part of England has 

 been so thoroughly investigated. 



The first thing that strikes one in recording the mollusca for 

 any given area, is the extremely limited range of certain species 

 when, as far as we can judge, there is no visible cause for the 

 limitation, though certain well-known facts will account for 

 a good deal ; as, for example, the nature of the soil, whether 

 calcareous, clay, or sand, and consequent on this the prevalence 

 of certain food-plants. Yet when all these are taken into con- 

 sideration, there still remains very much to be explained. 



There are several questions which come up in writing a 

 paper of this kind which are very difficult to answer : as, why 

 certain species which were common in one year afterwards 

 gradually became rarer, till they either died away altogether, 

 or after a lapse of a year or two suddenly reappeared in 

 profusion ; and why others, as, for example, Clausilia biplicata 

 at Putney, should only be found in a space almost to be 

 measured in square yards ; while another closely allied species 

 is common throughout the whole of England. But this is not 

 to be a paper on the problems of distribution, extremely inter- 

 esting though they are. 



Before going into any details, and assuming that some of 

 those present are not very familiar with the subject, I will go 

 rapidly through the genera with which we have to deal to- 

 night. Beginning with the Freshwater Shells, out of 46 

 species only three are absent. First among the bivalves is 

 SphcBrium, which includes four species, three of which are 

 found in the district ; the other Sph. ovale, Fer., is known only 

 from a few places in the north of England. I have one 

 specimen from the Grand Junction Canal, but have never 

 heard of any being found there lately. Pisidium, five species 

 distributed throughout ; but they are very small shells, 

 lamentably deficient in constant characteristics, and merge so 

 into one another by insensible gradations that very few people 

 can accurately determine them. Unio, comprising three 

 species, one absent, U. margaritifer, L., the pearl-bearing 

 mussel, which is only found in rapid streams in the mountainous 

 districts of the north of England, Wales, Ireland and Scotland. 



G 



