GASTEROPODA. 



89 



These shells are remarkable for the different appearance 

 they present during the several stages of their growth. 

 Whilst young they are very thin, almost 

 colourless, and dull on the outside ; the mouth 

 is rather wide, the outer lip not rolled inwards, 

 but having a sharp edge. At a more advanced 

 period the lips are curved inwards; but the 

 shell is still thin, and the colours not bright. 

 The gradations of advancing growth may be 

 easily remarked in the common (7. Manritiana. 

 At a later stage, when it has attained its 

 full growth, the outer lip is rolled inwards 

 and thickened, the back marked with the most beautiful 

 patterns, and bearing an exquisite polish, and the spire, at 

 the same time, if not entirely hidden, scarcely projects. 

 The animal itself undergoes a considerable change in ap- 

 pearance during its growth. Its mantle is at first small, 

 but increases with the enlargement of the shell, and ex- 

 pands at its sides into two large wings, which are generally 

 covered with ee warts, spinous processes, forked, tufted, or 

 ramified filaments, or tubular papillae." From these is 

 deposited the final polished layer of shelly matter, which 

 forms the exterior coating, and which is caused by the 

 animal enveloping the shell entirely in its mantle. At the 

 point where the two expansions of the latter meet on the 

 back of the shell a line is consequently formed, which is 

 called the dorsal line ; but occasionally these lobes are so 

 much extended as not only to meet, but to wrap over each 

 other, in which case there is no line. Forbes says, — 

 " The difference of aspect between the mollusks when 

 crawling, with their beautifully coloured soft parts ex- 

 posed, often completely concealing their enamelled shells, 

 and their appearance when, after being seized, they sud- 

 denly and instantaneously withdraw their bodies and 



