114 UNIVALVES. — TROCHUS. 



vivid play of iridescent colours which it exhibits when held 

 in different positions and lights. 



The T. Cookii, from Cook's Bay, has its aperture closed 

 with a horny lid or operculum, to secure the inhabitant 

 when retired within its shell. This appendage is most 

 frequent among the Univalve genera; it is affixed to 

 the animal, which, as it retreats into the spiral whirls of its 

 shell, draws the operculum in along with it, to a particular 

 situation of the aperture, where, from the extreme accura- 

 cy of its adjustment, it perfectly closes the orifice, and 

 thereby forms a complete barrier against any outward at- 

 tacks. 



The operculum 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 perfect- 

 ly smooth, and others strongly granulated. 



The T. conchyliophorus is a very curious and remark- 

 able species; for it is invariably covered with other sub- 

 stances, strongly adhering to the whirls of the shell. 



Of this species there are two distinct varieties: one is 

 called the Conchologist, from its being loaded with frag- 

 ments of shells, and other testaceous substances ; and the 

 other is very properly named the Mineralogist, as its bur- 

 then 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 great- 

 ly 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 prominent 

 species are the T. telescopium and T. dolabratus. 



The greater part of the Trochi present a brilliant mo- 



