THE GEELONG NATURALIST. te] 1 
on the right side, and then the whorls are turned to the left, The 
growth of bivalves is carried on in the same way, each valve being 
simultaneously enlarged, as will be seen in most gueia Nhe lines 
of growth being easily traced. 
The power of forming shell is not confined to the edge of the 
mantle, but exists over its whole extent; this is proved by breaking 
the shell of a living snail at a distant from the edge or lip. The 
animal at once sets about filling up the gap, and effects it in the 
course of a few days by a succession of liquid shelly matter, which 
exude from that portion of the mantle which has been laid bare. 
The regularity of the growth of many shells appears to be 
interrupted at certain times by changes in the shape of the mantle; 
this is seen in many species which exhibit knobs or spines rojecting 
from the surface, and which is produced by the mantle altering its 
form, and dividing into thick or slender branches. On these 
processes are formed hollow spines, which are gradually filled up 
by successive depositions of shelly matter, and eventually become 
solid; the mantle being then withdrawn from them proceeds to 
form new shell, branching out at regular intervals into fresh rows 
of spires, knobs, or foliations, and in this manner are produced the 
spines, ridges, &c., seen on a great many shells. In some species, 
as the Spider shell, the change in the mantle does not happen until 
the mollusca arrives at its full growth. The secretion of shelly 
matter not being called into action for the purpose of enlarging the 
shell, if it is continued it only adds to the thickness of the shell, 
without extending its dimensions. The cowrie when young, has a 
thin sharp lipped shell; in the adult state the lip is curled in and 
thickened. — Some sheil-fish have the power of dissolving the inner 
portions of the shell. When the body requires more space, those 
whose spires are exposed (and being thin in their young state would 
be liable to be broken by the action of the sea) have that part 
strengthened by the internal deposition of shelly matter. Some, 
few of the mollusca, withdraw their body from the upper portion of 
the spire, this portion then decays, and gets broken off. Many of 
the turbinated mollusca have the power of constructing a kind of 
door to the entrance of their shells, this is: called the operculum; 
these can always be found in hundreds arounds our coast; they look 
like shell buttons, but not perforated; there are the opereult of 
the common (turbo undex]ata) so plentiful at Bream and Spring 
Creeks. The beautiful and varied colours which shells presents are 
produced by colouring matter contained in the pores which exists 
in the front part only of the mantle, and like matter is made to 
penetrate the substance of the shell before it beeemes hardens; this 
