CONCHIEERA OR BIVALVES. 47 



teeth, D D. EE are the muscular impressions, 

 and F the pallial impression, which marks the 

 size of the animal, being formed by the border 

 of the pallium or mantle. G G are the sides 

 of the shell, which is called an equilateral shell 

 when they are equal in length, and inequilateral 

 when they are unequal. H is the ligament, 

 which is horny, and by its elasticity tends to 

 keep open the valves. This is counteracted at 

 the animal's pleasure, by the muscles attached 

 within the shell. The ligament is sometimes 

 internal, and sometimes external, or both at 

 the same time. 



In describing a bivalve shell, both Linnaeus 

 and Lamarck place the shell on the bosses, 

 with the opening above and the ligament be- 

 fore ; the valves are either called right or left, 

 as they are nearest to the right hand or left 

 hand of the observer. 



Order I. 



BRACHIOPODA.* 



Authors have been much divided in opinion 

 about the order Brachiopoda, not only as to its 



* Les Brachiopodes, Lamarck. 



