190 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
becomes very strongly developed. From it a cuticular 
structureless membrane passes off over the free edge of the 
shell forming the epicuticula or periostracum (Epic.). 
When the shell is dissolved off in acid, this cuticular 
investment is seen to be really continuous with the mantle 
edge, so that the latter is firmly attached to the outer 
surface of the shell. 
On the inner surface is a ciliated zone which begins a 
little way back from the free edge of the inner fold, and 
ceases or begins to die out opposite to the line of attach- 
ment of the pallial muscles to the shell. Coincident with 
the distribution of the cilia is that of a zone of mucous 
glands opening on to this ciliated surface on the mantle 
margin. These glands are very similar to those which 
will be described as present on the ciliated tip of the 
foot (p. 194), but are, as a rule, unicellular, the body and 
conducting stalk being formed from a single cell. Occa- 
sionally several cohere together forming a structure of the 
same nature as those found on the foot, but they are very 
generally much smaller individually. Their stalks passing 
out through the epidermis give the latter an irregular 
appearance. The tips of these stalks project out among 
the cilia as prominent little knobs. 
The pallial muscles (Ret.m., fig. 23) have a narrow zone 
of attachment to the shell (Ret.m’., fig. 10). Here the 
epidermic cells disappear completely from the mantle 
surface. 'The muscle bundles, passing ventrally, parallel 
to the surface, are attached to the shell at a very acute 
angle. The bundles run along the outer surface for a 
short distance, then divide into two series. One of these 
continues to run along the outer surface, and terminates 
in the connective tissue in the outer and median folds, 
the other series, consisting of stronger bundles, crosses over 
to the inner surface and breaks up into smaller bundles, 
