38 



CLASS III. CONCHIFERA. 

 (BIVALVES.) 



A BIVALVE may be considered as a Tunicate 

 more or less opened down one side (the ventral 

 side), and enveloped in a shell instead of a test. 

 Hence the shell (representing the test) , the mantle, 

 and the breathing organ, instead of being so many- 

 sacs, are all composed of leaves facing each other, 

 opening in front, and united at the back, exactly 

 like the covers and leaves of a book. 



The shell is the outermost envelope. It consists 

 of a pair of valves, more or less exactly corre- 

 sponding to each other in size and shape. Its 

 substance is carbonate of lime deposited in mem- 

 branous cells, or in membranous folds. By means 

 of acids the lime may be wholly dissolved away, 

 and the membranous base remains, retaining the 

 form without the consistence of the original shell. 



Considered as to its structure, shell consists of 

 two very distinct portions. The outer layer is 

 formed of close prisms of lime, or rather cells filled 

 with lime, standing perpendicularly or obliquely 

 to the plane of the valve. They are deposited 

 at certain definite periods by the mantle-borders, 

 in a manner to be described presently. The inner 

 layer is more or less pearly, and is composed of 

 thin coats of lime, not contained in cells, but 

 deposited in folds of the membranous base. The 

 origin of this layer is the whole outer surface of 

 the mantle-leaves. 



The shell-valves are not united to the contained 

 animal, except at certain points by the insertion of 



