GASTEROPODS 345 
umbilicus is fairly large, its external opening, in the adult shell, 
has been almost completely closed by a callous process thrown off 
from the inner or columellar lip. 
Gasteropod shells may be sinistral or dextral, according as the 
whorls turn to the left or right. The great majority of marine 
species are dextral, having the opening on the right, although a 
few species exhibit the curious property of being constructed upon 
either the one or the other plan without apparent reason. Sinistral 
specimens of many normally dextral species have been discovered, 
but they are so rare as to warrant the belief that such specimens 
are merely deformities or monstrosities resulting from some acci- 
dent of birth. 
The collector will soon learn to distinguish between young and 
adult specimens, for those finishing touches of wonder and beauty 
which the mollusk gives to his protective covering are made only 
by the adult. The outer lip of young gasteropod shells is usually 
thin and fragile; even if slightly thickened, it has an unformed 
or unfinished appearance. 
Shells differ greatly in their structure, many species being por- 
celanous like china, others glassy, and many more are of a 
softer chalky composition. In the latter case the shell is usually 
covered with a thick membranous skin, which, when removed, 
leaves a dull, lusterless, white body beneath. Shells of this tex- 
ture, when cast upon the beach, soon yield to the weathering 
action of sun and air. The porcelanous shells are composed of 
successive layers of carbonate of lime, throughout which is a 
filmy membranous framework of a substance similar to that of 
which the covering of the Crustacea is formed. The mode of 
deposition of the various layers of crystalline calcic carbonate 
and the peculiar lineation of their outer surfaces give rise to the 
iridescent or nacreous appearance of some shells. 
With the exception of those species in which the mantle of the 
animal is extended over the. edge of the aperture and more or 
less envelops the entire test, there is an outer skin of membra- 
nous or organic matter which overlies the surface of the shell. 
This skin is sometimes quite thick, often hairy, and is usually of 
a dull greenish- or brownish-yellow color. In many genera this 
