MOLLUSCA OF INDIA. 189 
tion, though it can only be a modification of the same inherited 
character. { have endeavoured from examination of the animal and 
shell in these two tube-bearing genera to understand how this 
difference can be produced, and on Plate LI. are figures and sections 
presenting internal and external views of these parts. Figs. 6a 
and 9a represent not the shell, but an ideal section through the 
mantle-margin and respiratory tube (fig. 7, ¢). In fig. 9a the 
thick line represents the early stage, the dotted line the last stage of 
growth in Lthaphaulus. 
It is easy to understand that any slight folding of the mantle, 
either on its internal or external surface, will produce a notch upon 
the peristome, such as we find in Pterocyclos, and that if this fold be 
increased so that the lower edges meet, a more or less circular notch 
or circular orifice will be produced, which, still retaining the shell- 
secreting property of the mantle, would deposit a tube. Then 
according as the mantle on its free curvilinear margin grew faster 
or slower than the tube or part passw with it, so would the shelly 
tube be modified in form and position; or, again, the tube may 
increase or remain constant in length. Thus I imagine in Rhaphaulus 
the complete curvilinear edge of the mantle is slightly anterior to 
the free end of the tube, which has a continuous growth from its 
first formation, and thus the outer surface of the shell is formed 
over it up to the aperture, rendering the tube internal and its orifice 
posterior. At this stage the mantle-edge ceases to increase, the 
animal having reached its maximum development ; this is stationary 
so to speak and the thickened peristome is then formed. The inter- 
nal tube still continuing to grow at the free end passes upwards, 
backwards, or downwards externally, the common specific character. 
In Alyceus the tube would appear to have a short but permanent 
length and position, and from the commencement of its formation, 
or folding in of the mantle-edge, is turned upward and backward 
over the edge of the peristome and slightly in advance of it, deposit- 
ing the shelly tube externally upon the outer surface of the body- 
whorl and in the sutural angle, and is never internal. It should be 
noticed that the length of the tube in Alycceus bears invariably a 
constant relationship to the swollen, close, regular, and strongly 
ribbed portion of the body-whorl, and there is some intimate con- 
structive connection between the two, indicating, I think, an inter- 
mittent action upon the line of the mantle-edge, during which it 
undergoes a change somewhat like the following. The part produced 
into a tube-like form is withdrawn, or reduced in size, on the forma- 
tion of a rib which may be likened to an incipient peristomatic lip ; 
on this being completed and the work of the mantle reduced, the 
fleshy tube is again produced, reflected over the margin to deposit a 
ring of shelly matter on the sutural tube, and then again retires 
during the formation of the next peristomatie ring. 
When the animal approaches its mature size the fleshy tube 
becomes atrophied, leaving a mere scar on the mantle corresponding 
with the internal orifice, and here the above ribbing or close costu- 
lation ends, ‘The portion of the whorl in front would then appear 
