50 
SPECii';^. 
narrowing impressions from the margins of the present scars, converg- 
ing towards the nmbones, and showing the process of growth and 
the successive shiftings of the muscles. 
The Pallial-line, which in tlie Integripalliate species follows more 
or less the outline of the ventral margin of the shell, is caused by the 
impressions of the points of attachment of the muscular fibres of the 
mantle, and indicates the line of attachment of the mantle to the 
shell. The siphonal impression or pallial sinus only exists in those 
shells with long and retractile siphons, its depth being an index of 
their length; the small siphons of Sp/ucrium cause little or no inflec- 
tion of the pallial-line. 
Species. 
Prior to the enunciation of the doctrine of evolution, the definition 
of species was simple and clear, as they could be described as the 
whole of the individuals descended from the pair originally created ; 
the general organization and form of each species being considered 
to be immutable and as fixed by the Creator. The universal accept- 
ance of the principles of evolution compel us to regard species as 
mobile and plastic, and although their characters may be modified by 
the responsive reflex action of the organism adapting itself more or 
less readily to the varying conditions of the environment, yet for 
all practical i)urposes specific characters can be regarded as stable 
and fixed, as any radical structural changes induced by the action 
of the different agencies of the environment, re(pure lengthened 
periods to modify a suflicient bulk of individuals to firmly establish 
permanent and suflicient differences for specific distinction. 
A species, therefore, may now be defined, as all the individuals 
which bear such resemblance, not only to their parents and offspring, 
but to each other, as to make it reasonably certain that they belong 
to one common stock. This assemblage of individuals should also 
be capable of inter-breeding amongst themselves, produce fertile off- 
spring, and should not be too closely connected by intermediate 
forms with other species or groups of individuals. 
The general form of the shell, and the arrangement and peculi- 
arities of the sculpture and colouring furnish the principal testaceous 
characters of species in the Gastropoda ; in the Pelecypoda, the 
difl'erent forms of the shell, the position of the umbones, the char- 
