MOLLUSCA. 271 
The margin of the shell on which the ligament and teeth 
are situated is termed the hinge-line. 
Lamellibranchs breathe by four plate-like gills (whence 
the name), two on each side underneath the mantle (Fig. 
78). In the higher forms, the mantle is rolled up into 
two tubes, or siphons, for the inhalation and exhalation of 
water. They feed on microscopic organisms filtered from 
the water. A few are fixed; the Oyster, e. g ., habitually ly- 
Fig. 226—Lamellibranch ( Mactra ): a, foot; 6, c, siphons. 
ing on its left valve, and the Salt-water Mussel hanging to 
the rocks by a cord of threads called “byssus;” but most 
have a “foot,” by which they creep about. Unlike the 
Oyster, also, the majority live in an erect position, rest¬ 
ing on the edges of their shells. Over four thousand 
living species are known. These are fresh-water and 
marine, and range from the shore to a depth of a thou¬ 
sand feet. 
The chief characters for distinguishing Lamellibranchs 
are the muscular impressions, 139 whether one or two; the 
presence of a pallial sinus, which indicates the possession 
of siphons; the structure of the hinge, and the symmetry 
of the valves. (Fig. 99). 
The following are the leading types of structure, as 
shown by the shells: 
1. Monomya: with one adductor muscle; no siphons,* 
foot wanting, or very small; shell unequivalve and eden- 
