100 



affixes itself to rocks, and remains suspended, notwith- 

 standing the magnitude and weight these shells some- 

 times attain, having been found weighing five hundred 

 pounds, and measuring several feet in length, a size 

 unequalled by any other testaceous body. The ani- 

 mal is said to produce very fine pearls, but there is no 

 nacreous appearance on the valves. Shell regular, equi- 

 valve, inequilateral, transverse, posterior slope heart- 

 shaped and widely gaping: hinge with two teeth, 

 compressed, unequal, and inserted; valves with broad 

 rounded longitudinal ribs, armed with scales more or 

 less vaulted; ligament exterior, and placed on the 

 longest side. 



Tridacna gigas Tridacna crocea 



elongata mutica 



squamosa serrifera 



[And one fossil species.] 



H1PPOPUS. 



chama hippopus. — Linn. 



Sowerby's Genera, No. 13, pi. I. Jig. 1. 



Lamarck has distinguished this from the preceding 

 genus, (which it nearly approximates), from the cir- 



