30 Lamarck's Genera of Shells. 



since the same is nearly the case with the chamacea, they are not 

 common to all of them ; for some, as the pectines, &c., have a 

 regular shell, without a distinctly-foliated texture, others, as the 

 lingula, have their valves equal, or very nearly so. 



This order consists of three sections, the first containing three 

 families, the second and third, two each. 



Section 1st. 

 Ligament marginal, elongated on the edge, sublinear. 

 Most of the shells of this section adhere to marine substan- 

 ces by a byssus ; several of them are equivalve, not foliated. 



1st Family. 

 Tridacnea, (2 genera.) 



Shell transverse, equivalve, muscular impression below the 

 middle of the superior margin, and extending, on each side, 

 under it. 



The shells of this family are regular, solid, and remarkable 

 by their sinuous or wavy superior margins. 



1. Tridacna*. 



Shell regular, equivalve, inequilateral, transverse ; lunula 

 gaping. Hinge with two compressed, unequal, anterior, enter- 

 ing teeth. Ligament marginal, external. 



Linnaeus confounded the tridacnse with the chamee. They 

 are rather handsome shells, often above the middle size, and 

 sometimes so gigantic, that one species, (T. gigas,) is the largest 

 shell known. 



The animal has but one transverse muscle, and the interior 

 of the shell exhibits a single, elongated, arched, muscular impres- 

 sion, running below the superior limb, and widest at the middle 

 of the margin of the valves. 



The tridacna is perfectly distinguished from the hippopus, by 

 the lunula always being open and gaping, through which the 

 animal protrudes a byssus, to fix its shell to the rocks, and by 



* From TjEif, three, and Jaxva, to bite. A name given to a kind of oygter, 

 solarge as to require to be eaten in three pieces.— Plin. 32. 6. 



