CLASSIFICATION 31 



to those in which the umbones are straight, " ortho- 

 gyre," or are directed backwards, " opisthogyre." 

 The last-named condition may be the pelecypod 

 equivalent of hypertrophy in the Gastropod shell. 

 More often than not the two valves are of equal size 

 and shape, and the shell is " equivalve," as in the 

 Cockle ; sometimes, however, as in the Oyster, one 

 valve is smaller than the other, and the shell is 

 " inequivalve." The Bivalves are all more or less 

 " inequilateral " — that is to say, if a line be drawn 

 from the umbo to the ventral margin of the valve, 

 the portion on one side of the line, usually the front 

 one, will be found to be smaller than that on the 

 other. When the shell is shut it is said to be 

 "close " if the valves fit accurately, and to "gape " 

 if openings be left (Plate XVII., Fig. 5). 



Near the umbones the two valves are united by a 

 chitinous " ligament " (Plate XVI., Fig. 2, /), formerly 

 known under the misleading name of " cartilage," 

 which is made up of an outer non-elastic layer and 

 an inner, fibrous, elastic layer. The whole forms a 

 sort of C spring which tends to open the valves, the 

 act of closing being effected by two powerful 

 "adductor" muscles that pass from one valve to 

 the other, and are situated at either end of the axis 

 of the body. 



To prevent the opposed valves from shifting when 

 closed, a series of projections, or " teeth," fitting 

 into each other, are developed near the ligament. 



