GUPPY — MOLLUSCA OF TMXTDAD. 
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Water mollusca I have found in the island, and which havo 
been enumerated and partly described in the memoirs pre- 
viously alluded to. In compiling this catalogue it has been 
my aim to present not merely a dry list of names, but to 
give also such details that the species may readily be iden- 
tified, even by persons who have little opportunity for the 
determination of shells. Some of the principal synonyms 
and authorities are also added. 
Ono of the most obvious distinctions to bo noticed 
amongst the terrestrial and fluviatile mollusca is the pre- 
sence or absence of the operculum or shelly plate closing 
tho aperture of the shell. As it has been found that the 
presence or absenco of this organ is of great importance in 
classification, and indicates very precisely the group to 
which the animal belongs, wo shall use that character to 
divide the species into two groups ; and in the second place 
we shall divide each of these groups into two by their 
“ station,” that is, as to whether they inhabit the land or 
tho water. Wo si 1 all thus, leaving out of view the single 
bivalve of tho collection, havo four main groups to which to 
refer our shells. 
The characters of tho genera and species are drawn from 
tho shells as being the part of the organism most accessible 
to observation. The study of tho soft parts is of course 
just as necessary to enable us to understand the true affini- 
ties of tho animals, but where tho characters afforded by 
tho sholl are sufficient for identification they seem to be 
preferable for use in diagnosis. 
Tho few shells afforded by our island, of which some are 
difficult to bo found, can of course give no idoa of the im- 
mense number and exquisite beauty and variety of form of 
tho terrestrial mollusca of some countries, such for example 
