III.] INTRODUCTION. Ivii 



it is entirely wanting. M. nivalis has a distinct operculum ; 

 while M. Ginnaniana (which belongs to the same section 

 of this genus and is found in company with M. nivalis 

 in our northern sea) has not the slightest vestige of an 

 operculum . Exceptional instances of the same kind occur 

 in the tropical genera Valuta, Conus, and Oliva. But a 

 remarkable peculiarity is presented by some species of 

 lanthina, which are furnished in their embryonic state 

 with perfect opercula. These processes afterwards dis- 

 appear, being probably absorl^ed by the animal when the 

 shell becomes too large to be thus closed. Sars, Van 

 Beneden, and Vogt have shown that the fry of many 

 Nudibranchs, as well as of that anomalous mollusk 

 Elysia viridis, have Nautiliform and operculated shells. 

 The fry of Dolium perdix, which has also an operculated 

 shell in this stage of growth only, is so unlike the adult, 

 that the late Professor Forbes constituted for it a new- 

 genus of another order, under the name oi Macgillivrayia. 

 The fry of a curious land mollusk [Parmacella), which 

 partakes more of the nature of a slug than a snail, is said 

 to be enclosed in an operculated shell. In the adult this 

 shell becomes more rudimentary and only covers a small 

 part of the body. This is a case of retrogressive, rather 

 than of " progressive development." 



Epidermis. — Most shells have an outer horny covering, 

 called an " epidermis " or ^' periostracon," which appears 

 to be analogous to the periosteum of bones in vertebrate 

 animals. Its office is probably to protect the shell from 

 the chemical action of the air or water inhabited by the 

 mollusk. It is formed simultaneously with the shell, and 

 probably by the same organ of secretion. It is usually 

 glossy, and sometimes resembles a coat of varnish. In 

 Astarte (a genus of marine bivalves) it is thick and 

 strong. In some of the whelks and land snails it re- 



c5 



