40 Land and Freshwater Mollusks. 



viduals of each species. About 120 species are at present 

 determined as belonging to Great Britain and Ireland, some 

 authors counting a few more, some less, according to their views 

 of specific difference. It is doubtful whether any of these are 

 peculiar to our fauna — one species of Zonites was so considered, 

 but it seems to be identical with specimens found in the north of 

 France ; the Amphipeplea, a freshwater species allied to Limnaeus, 

 discovered in an Irish mountain lake by the late Dr. Ball, has not, 

 as far as I am at present informed, been identified elsewhere, but 

 it is held by some to be not a distinct species but merely a variety. 

 There may remain, and doubtless there do remain, other species 

 yet to be found, and it may be the good fortune — a naturalist's 

 peculiar pleasure — of some now present to make discovery of 

 forms yet undescribed as British. 



Although in comparison with British Entomology and Botany 

 and even with Ornithology, the field of research to which I draw 

 your attention is limited, yet the objects themselves have that 

 special interest which is kindled from the consideration that there 

 is yet much to be learnt I'especting them, much to be discovered of 

 their structure, the conditions of their life and growth, the rapidity 

 or retardation of their development, their food, the effects upon 

 them of climate and generally of unusual circumstances of life, 

 the limits of variation of what are considered their specific 

 differences, that subject which in all animal life is so interesting 

 and yet so full of what we must confess to be still mystery to our 

 present knowledge ; above all, are yet to be known the laws of 

 their distribution. 



It does not seem to me irrelevant to indicate these difficulties — 

 rather I would say these prizes of natural knowledge to be won — 

 because I am persuaded that the most abstruse of such questions 

 will yet yield to accurate observation and patient thought, that inter- 

 preters will yet be found for many pages of the great open book 

 of nature. 



I would however touch, with your permission, on a few minor 

 points respecting which, observation of our land and freshwater 

 shells is needed. I pass over questions of structure as these 

 seem to demand much leisure and perhaps some apparatus for 

 their investigation, but allow me to recommend collectors to be on 

 the watch among the most common species of our shells for curious 

 and often most interesting departures from ordinary form. Reversed 

 shells, that is shells with the spire, the central support round 

 which the shell is, as it were, built — twisted in a sinistral instead 

 of a dextral direction, or vice-versa, are to be met with. Several 

 fossil shells exhibit this character, and also several foreign species 

 of Helix and Bulimus, the sinistral twist being in these cases the 



