36 
THE SHELL IN BIVALVES. 
The Pelecypoda or Bivalves, also known as Lipocepiiala and 
Lamellibranchiata, are exclusively aquatic mollusks, and differ 
markedly from the Gastropoda, not only in their shell, but in many 
points of their organization, and rank next that group in the number 
and variety of its species and genera, though individually the 
Pelecypods are relatively the most numerous. 
The shells of Bivalves are typically composed of two distinct and 
convex or datly conoid pieces or valves, which are ap})lied to the 
right and left sides of the animal, and in the dimcions species, as in 
the univalves, the shells of female individuals are said to be notice- 
ably shorter and more ventricose than those of the males. 
'I'he line along which the valves are joined together by the ligament 
is called the hinge-line, and is placed on the dorsal region of the 
animal, forming the Upper or Dorsal Marhin of the shell ; the 
side op[)osite the hinge is the Ventral or Jjower Margin and is 
always more or less thin and sharp ; the end upon which the liga- 
ment is situated is the Posterior or Siphonal Margin, the orifices 
of the hraiichial or iiicurrent and the anal or excnrrent sijihons being 
