MOLLUSCA. 271 



The margin of the shell on which the ligament and teeth 

 are situated is termed the hinge-line. 



Lamellibnmchs 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- 



. 226 Lamellibrauch (Mactra) -. , foot ; b, 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- 



