132 POPULAR BRITISH CONCHOLOGY. 



The Top-sliells, so named from their form, which is conical, 

 are ahnost all brilliantly iridescent within the aperture, 

 the same kind of pearl lining their inner surface as adorns 

 the Uniones and the Pearl Oysters. The whorls are gene- 

 rally more or less beautifully sculptured ; some with coro- 

 nated edges, some with spiral headings, others with symme- 

 trical undulations, and others with these adornments com- 

 bined. The colours are generally bright and clear, but 

 have their glories veiled, as well as preserved, by the brown 

 epidermis which covers them. 



The foot of the Trochidm is generally expanded and lobed 

 at the sides, and presents the first instance we have met with 

 in our course, of carrying near its hinder part a little plate, 

 called the operculum. It is so arranged that when, from a 

 desire to rest or to escape from sudden danger, the animal 

 finds occasion to retire within his stronghold, he first throws 

 back his head, with its eyes and tentacula, withdrawing 

 them under the mantle which lines the aperture; then, 

 gathering up and drawing in the lobes and cilia of his body, 

 he brings the front part of his foot over them ; and last of 

 all comes the hinder part of the foot, enclosing the rest, and 

 bringing with it the operculum, which, fitting neatly to the 

 aperture, shuts all in safe, bidding defiance to external foes. 



