A 
MONOGRAPH 
OF THE 
LAND AND FRESHWATER MOLLUSCA 
OF THE 
BRITISH ISLES. 
Definition of Conchology. 
Concliology, tlie term by which the study of the mollusca is most 
generally known, is a combination of the Greek words, Koyxi] (a shell- 
fish), and Ar;yos(a discourse or treatise); and is usually understood to 
embrace the study of the complete subject — not merely the shell, 
hut also the animal which forms it. By some authors the term has been 
restricted to the study of the shell only, and the word Malacology 
used to designate the investigation of the animal or soft parts ; 
although Tryon and some other scientists have proposed that Malaco- 
logy shall be understood to embrace the complete subject, and super- 
sede the older name. 
The molluscan sub-kingdom embraces those organisms with soft 
and fleshy bodies, enclosed or covered by a muscular sac which is 
called the mantle, and which usually secretes a more or less regular 
and .symmetrical shell, mainly composed of carbonate of lime, the 
chief function of which would appear to be the protection of the 
vital organs of the body. In some genera the shell is internal or 
concealed beneath the mantle, it is then usually of a simple flattened 
plate-like form or even reduced to a few granules. 
The Mollusca are distinguished from the Articulata, by their bodies 
not being segmented, and also by their nervous system consisting of 
