276 POPULAR BRITISH CONCHOLOGY. 



begins within, and terminates a little in advance of, the one 

 before it. 



A shell consisting of one piece is called a JJnwalve ; as 

 a snail or a periwinkle. 



A shell composed of two pieces opposed to each other, 

 i^ called a Bivalve ; as a mussel or an oyster. 



A shell made up of several pieces in a series is called a 

 Multivalve. Now that the Cirrijjedes are no longer in- 

 cluded in the class of mollusca, the only MuUivalve is the 

 Chiton. 



These definitions must not however be understood in 

 their strictest sense, or without qualification. Tor, while a 

 Univalve is said to consist of a single piece, it must be re- 

 membered that, in many cases, the opening of the hollow 

 in that piece is enclosed by a movable door, which, fixed 

 on the foot of the animal, is brought in upon it when a 

 condition of rest is assumed. This is the operculum, which 

 is quite accessory to the principal shell, and does not, like 

 the opposite valve of a bivalve, turn upon it by a hinge. 

 Similar observations may be made respecting some bivalves. 



The PJwlas, for instance, besides the pair of valves com- 

 posing the main shell, has, in many instances, one or more 

 separate smaller pieces named accessor^/ plates, which are 



