394.) Cause of Folds; Shell of Gasteropoda. 911 
the body of the shell near the posterior commissure will mark 
the special conditions in that part of the aperture. 
When the secreting surface is thus wrinkled or corrugated 
longitudinally the wrinkles and ‘the concave folds between 
them will be directed in the sense or direction in which the 
body moves in emerging from or withdrawing to the whorl. 
The summits of the convex wrinkles will be appressed more 
or less forcibly against the shell-wall exterior to them in 
which they are contained. The semi-fluid, living secretion of 
which the shell-lining is built up, exuding from the whole 
surface of the mantle, will be rubbed away from the lines of 
the summits of the wrinkles and tend to accumulate in lines 
‘corresponding to the concave furrows between the wrinkles. 
This secretion hardens rapidly, and these lines would become 
somewhat elevated ridges which would by their presence 
(when once initiated) tend to maintain the furrows and 
wrinkles in the same place with relation to the thus-initiated 
liree, as these elevated lines are called when on the outer lip; 
or plaits, when situated on the pillar. 
The modification referred to generally takes place during 
resting stages of the animals’ growth, since while the animal is 
rapidly extending its coil the secretions seem to be concentrated 
along the mantle margin, while the general mantle-surface 
resumes its secretive function (or the latter becomes active) 
‘somewhat later, after the formation of a definite shelly varix, or 
thickened margin, indicating a resting stage in the animal's 
career. Itis probable also that during rapid growth there is 
less compression of the tissues than during the resting stages. 
The external sculpture and some of the modifications of the 
-aperture are connected with the functions of the extreme edge 
of the mantle; those we are at present considering relate more 
-especially to the function of its general surface by which the 
layer whieh lines the whorls, the pillar, plaits and lire are 
Solely secreted and deposited. 
In species with the adductor muscle attached to the pillar 
near the aperture the wrinkles would be fewer, and their 
action, if any, confined to the vicinity of the margin of the 
-aperture. The deeper the attachment the greater will be the 
