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 mauj 7 ' 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 



