HABITS AND ECONOMY OF THE MOLLUSCA. 13 
winter they retire to deeper water, and in the following spring 
return to the tidal rocks, attain their full growth early in the 
summer, and after spawning-time disappear. 
The land-snails are mostly biennial; hatched in the summer 
and autumn, they are half-grown by the winter time, and 
acquire their full growth in the following spring or summer. 
In confinement, a garden-snail will live for six or eight years; 
but in their natural state it is probable that a great many die 
in their second winter, for clusters of empty shells may be 
found, adhering to one another, under ivied walls, and in other 
sheltered situations; the animals haying perished in their 
hybernation. Some of the spiral sea-shells live a great many 
years, and tell their age in a yery plain and interesting man- 
ner, by the number of fringes (varices) on their whorls; the 
contour of the ranella and murex depends on the regular re- 
currence of these ornaments which occur after the same inter- 
vals in well-fed individuals, as in their less fortunate kindred. 
The ammonites appear by their varices, or periodic mouths 
(Pl. III., fig. 3), to have lived and continued growing for many 
years. 
Many of the bivalves, like the mussel and cockle, attain their 
full growth in a year. The oyster continues enlarging his shell 
by annual ‘‘ shoots,” for four or five years, and then ceases to 
grow outwards; but very aged specimens may be found, espe- 
cially in a fossil state, with shells an inch or two in thickness. 
The giant-clam (fridacna), which attains so large a size that 
poets and sculptors have made it the cradle of the sea-goddess, 
must enjoy an unusual longevity; living in the sheltered 
lagoons of coral islands, and not discursive in its habits, the 
corals grow up around until it is often nearly buried by them ; 
but although there seems to be no limit to its life (though it may 
live a century for all that we know), yet the time will probably 
come when it will be overgrown by its neighbours, or choked 
with sediment. } 
The fresh-water molluses of cold climates bury themselves 
during winter in the mud of ponds and rivers; and the land- 
snails hide themselyes in the ground, or beneath moss and 
dead leayes. In warm climates they become torpid during the 
hottest and driest part of the year. 
Those genera and species which are most subject to this 
‘‘ summer sleep” are remarkable for their tenacity of life; and 
numerous instances haye been recorded of their importation 
from distant countries in a living state. In June, 1850, a 
liying pond-mussel was sent to Mr. Gray from Australia, which 
