BIVALVE SHELLS. 



391 



In the first place, they are jointed by a hinge, which is in some 

 instances so firm and complicated, that it holds them together 



when all the soft 

 parts have been 

 removed. This 

 hinge is some- 

 times formed by 

 the locking of a 

 continuous ridge 

 on one valve into 

 a groove in the 

 other, and some- 

 times by a num- 

 ber of little pro- 

 jections or teeth, 

 which fit in to cor- 

 responding hol- 

 lows in the op- 

 posite valve. In 

 the neighbour- 

 hood of the hinge 

 (sometimes out- 

 side, sometimes 

 inside, or both), 

 is fixed the liga- 

 ment ; which is 

 composed of an 

 elastic animal 

 substance ; this 

 answers the pur- 

 pose of binding 

 the valves to- 

 gether, and at 



the same time of keeping them a little apart, which may be 

 regarded as their natural position. When the animal wishes to 

 draw the valves closely together, it does so by means of the 

 adductor muscle, which is fixed to the interior of both valves at 



FIG. 585. A, a, b, length of the shell ; c, d, height ; e, lunula, 

 above which is the summit ; d, the ventral or inferior edge. 



B, the line across marks the thickness of bivalves. 



c, a, anterior extremity ; 6, posterior ; c, d, muscular impres- 

 sions; ,/, palleal impression ; g, lower edge of the left valve. 



