CLASSIFICATION. 
11 
Tabular View of the Sub-Divisions of the Mollusca, vrranged 
TO shew their assumed Genetic Relationship. 
The groups printed in italics arc those not represented amongst the 
British Land and Freshicater Mollusca. 
MOLLUSCA. 
^ 
I I I I 
Cephalopoda Gastropoda Scaphopoda Pelecypoda 
Anisopleura Isopleura Heteromya Isomya Monoinya 
^ I 
1 I , I. . . I . 
Euthyneura Streptoneura Integnpallia Sinupallia 
I I ! I 
Pulmonata Opistkobranckiata Zygobranchia Azygobranchia 
Stylommatophora Basommatophora Pulmonata Pectinibranchiata 
or Pneumonobranchiata 
In all attempts at classification, it is necessary to carefully dis- 
criminate between species showing similarity of form and habit but 
not structurally conformable, and those in which the whole organism 
is in harmony with the external aspect and mode of life. The first 
show analogical relation only, but the latter are related homologically 
and should therefore be classified together. If the resemblances 
exist from a very early age, they are considered to be of the greatest 
importance and to imply community of descent. 
Almost every organ of the body has been proposed, at one time or 
another, as a basis for the better arrangement of the Gastropoda, 
the circulatory, nervous, reproductive, alimentary, and respiratory 
organs, as well as the presence or absence of the operculum, the 
arrangement and position of the eyes, etc., have all been or are now 
used separately or together for the purposes of classification. 
In the Pelecypoda, the muscular and respiratory systems have been 
strongly relied upon for chief divisions ; the characters of the hinge, 
with its interlocking teeth, and the development or absence of the 
siphonal tubes, have also, always been considered to be important 
features, but too much reliance should not be placed on any one 
character for taxonomic purposes, and confirmation should always be 
sought by other organs, as it will be readily conceded by everyone that 
all organic characters are of varying importance according to circum- 
stances, and this must of necessity be so in a natural classification 
based upon the general organization and affinities of the animal and 
