34 MANUAL OF THE MOLLUSCA. 
FORMATION AND GROWTH OF THE SHELL. 
The shell, as before stated, is formed by the mantle ; indeed, 
each layer of it was once a portion of the mantle, either in the 
form of a simple membrane or as a layer of cells; and each 
layer was successively calcified (or hardened with carbonate of 
lime) and thrown off by the mantle to unite with those pre- 
viously formed. Being extravascular it has no inherent power 
of repair.—( Carpenter.) 
The epidermis and cellular structures are formed by the 
margin (or collar) of the mantle; the membranous and nacreous 
layers, by the thin and transparent portion which contains the 
viscera; hence we find the pearly texture only as a lining 
inside the shell, as in the nautilus, and all the aviculide and 
turbinide. 
If the margin of a shell is fractured during the lifetime of 
the animal, the injury will be completely repaired by the re- 
production both of the epidermis and of the outer layer of shell 
. with its proper colour. But if the apex is destroyed, or a hole 
made at a distance from the aperture, it will merely be closed 
with the material secreted by the visceral mantle. Such inroads 
are often made by boring worms and shell, and even by a sponge 
(cliona), which completely mines the most solid shells. In Dr. 
Gray’s cabinet is the section of a cone, in whose apex a colony 
SSSA See 
SSS TAT 
Fig. 25. Section of a Cone perforated by Lithodomi, 
of lithodomi had settled, compelling the animal to contract 
itself faster than it could form shell to fill up the void. 
Lines of growth. So long as the animal continues growing 
each new layer of shell extends beyond the one formed before 
it; and, in consequence, the external surface becomes marked 
with lines of growth. During winter, or the season of rest 
