130 
AlIXILIAR Y ORGANS — EPIPIIR AG JI. 
ill tliis coiiiitiy, iiuicli more delicate and less iiiteniiiiigled with cal- 
careous particles than that formed for winter protection, being often of 
exceeding tenuity and transparency and heantifiilly iridescent. There 
is an opaipie and usually very ajipareiit white calcareous spot opposite 
the orifice of the respirator}- chamber, through which there often 
passes an opening or slit in the film, which though not invariable in 
its direction, is usually jiarallel with the outer margin of the shell. 
The point where the animal was last in contact is often somewhat 
puckered and irregular, and, like the circumscribed area ojiposite the 
respiratory orifice, has a whiter ami more opaipie character, owing to a 
greater density of the calcareous matter at that place. Additional 
layers may be afterwards added which strengthen and thicken the film. 
'I’lie winter or hihernal epiphragm 
is always thicker and more solid 
than the ordinary one. In llelir 
pomatla it is very thick', strong, and 
calcareous, with an outward con- 
vexity, which seems to distinguish 
those species with bulky bodies. 
Animals with more meagre propor- 
tions relatively to their .shell often 
have the epiphragm deeply sunk 
in the aperture. Species with a 
Showing ,ho suongly calcareous winter j calcareOUS sliell aud tllOSe 
epiphragm m situ. 
destitute of a snhmarginal rib to the aiierture have very thin, delicate, 
and often more or less imiierfect ones, and all intermediate stages e.xist. 
If the weather becomes very severe the animal shrinks further 
and further into the recesses of the shell, forming additional 
epi[)hragms at short intervals from each other, these becoming more 
and more delicate ami transparent and less and less mixed with 
calcareous matter. I have myself counted as many as si.x of these 
septa in a Yorkshire specimen of aspersa. 
Some authors state that the eiiiphragm is only iiartially formed if 
the aperture of the shell is already partly closed by adherence to the 
shell of another mollusk or any other object, the epiphragm in that 
case being said to be secreted merely to close uj) the space not 
occupied by the oliject to which the mollusk is attached. I have, 
however, frequently verified in //c/or ((.y)ersa that the ei)iphragm is 
complete and extends over the whole aperture, and is not invariably 
the partial production it has been asserted to be. 
Fig. 288.— /rv//rz//(Z L., 
Faversham, Kent, 
Collected by ibe late Mis.s K. lb 1‘airbrass, 
