Shells with an oblong callosity on the base of the valve. 

 Etheria elliptica Etheria trigonula 



Shells without an incrusted callosity at their base. 

 Etheria semilunata Etheria transversa. 



TRIDACNA. 

 chama gigas. — Linn. 

 Mawc's Linnmts, pi. \%jig- I. 

 The TridacnEe were confounded by Linnaeus with the 

 genus Chama, which are irregularly formed shells, 

 with one tooth only, and always attached to other sub- 

 stances by the lower valve, from which these are most 

 distinct, being equivalve, the hinge with two teeth, 

 and the animal affixed to other bodies by a mass of fili- 

 form tendons: other distinctive characters equally ob- 

 servable, are, that the valves possess longitudinal ribs, 

 armed with broad vaulted scales at equal distances, more 

 or less elevated, the outward edges waved and interlock- 

 ing with each other; its posterior slope always gapes, and 

 is generally crenated, through the aperture of which 

 passes a bundle of tendinous fibres, by which the animal 

 h2 



