STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 30 
which corresponds to it, shells cease to grow ; and these periodic 
resting-places are often indicated by interruptions of the other- 
wise regular lines of growth and colour, or by still more obyious 
signs. It is probable that this pause, or cessation from growth, 
extends into the breeding season; otherwise there would be 
two periods of growth and two of rest in each year. In many 
shells the growth is uniform; but in others each stage is 
finished by the development of a fringe, or ridge (varia), or of a 
row of spines, as in tridacna and murex.—(Owen, Grant.) 
Adult characters. The attainment of the full growth proper 
to each species is usually marked 
by changes in the shell. 
Some bivalves, like the oyster 
and gryphea (Fig. 26), continue 
to increase in thickness long after 
they have ceased to grow out- 
wards; the greatest addition is 
made to the lower yalve, espe- 
cially near the umbo; and in the 
spondylus some parts of the mantle 
secrete more than others, so that cavities, filled with fluid, 
are left in the substance of the shell. 
The adult teredo and fistulana close the end of their burrows ; 
the pholadidea fills up the great pedal opening of its valves ; 
and the aspergillum forms the porous disc from 
which it takes its name. Sculptured shells, 
particularly ammonites, and species of rostel- 
laria and fusus, often become plain in the last 
part of their growth. But the most charac- 
teristic change is the thickening and contrac- 
tion of the aperture in the univalyes. The 
young cowry (Fig. 27) has a thin, sharp lip, 
which becomes curled inwards, and enormously 
thickened and toothed in the adult; the ptero- 
ceras (Pl. 4, fig. 3) develops its scorpion-like 
claws only wheu full-grown; and the land- 
snails form a thickened lip, or narrow their 
aperture with projecting processes, so that it fig, 27. Young 
is a maryel how they pass in and out, and how Cowry-i 
they can exclude their eggs (e.g. Pl. 12, fig. 4, anastoma; and 
Fig. 5, helia hirsuta). 
* Fig. 26. Sectian of gryphea mcurva, Sby. Lias, Dorset (original; diminished _ 
one-half) ; the upper valve is not much thickened; the interior is filled with lias, 
| Cyprea testudinaria, L., young. 
