84 UNIVALVES TROCHUS. 



is affixed to the animal, and as tliat retreats into the spi- 

 ral whorls of its shell, it draws the operculum in along 

 with it, till arrived at one particular situation of the aper- 

 ture or mouth; where, from the extreme accuracy of its 

 adjustment, it perfectly closes the orifice, thereby form- 

 ing a complete barrier against any outward attacks, that 

 may be made on the animal. 



The operculum often varies in shape, according to 

 the form of the mouth it has to close : in some instances 

 it is elongated, and has a horny appearance; in others it 

 is circular, and of a very compact testaceous substance; 

 some are perfectly smooth, others strongly granulated. 



The Trochus Conchyliophorus, or Carrier Trochus, is 

 a very curious and remarkable species; for it is invariably 

 covered with other substances, strongly adhering to the 

 whorls of the shell. There are two distinct varieties: one 

 of which is called the Conchologist, from its being loaded 

 with parts or fragments of shells, and other testaceous 

 substances; the second variety is very properly named 

 the Mineralogist, as its burthen consists of stones, earths, 

 pebbles, ores, &c. When the former variety is loaded with 

 corals only, it is called the Zoologist or Coral-carrier. 

 They are considered, when heavily laden, as rarities. 



Some species of Trochi are much elongated, and greatly 

 resemble screw or needle shells. Unlike the rest of the 

 genus, they have an exserted pillar; and, when placed 

 on their base, they fall on one side. The most promi- 

 nent species are the Trochus Telescopium, or Telescope 

 Trochus, and the Trochus Dolabratus. 



The greater part of the Trochi present a brilliant mo- 

 ther-of-pearl, when uncoated; others have only their aper- 

 ture pearly or silvery; whereas some again are of a hue 

 something resembling bronze. 



