82 



TRIDACNA.— CHAMA.— ISOCARDIA. 



This family is divided into Tridacna, Chama, Isocardia, &c. 

 27. The Tridacna have an' equivalve shell, which is elon- 

 gate and gaping in front, or its edges are dentate {fg. 103). 



Their internal orga- 

 nization is remarka- 

 ble in several particu- 

 lars. The valves have 

 only a single adductor 

 muscle. The mantle 

 is widely open in front, 

 to give passage to 

 the byssus, and a 

 little beneath the an- 

 terior angle presents 

 Fig. 103.— TRIDACNA. another opening, by 



which water is conveyed to the branchiae; and the third open- 

 ing, which corresponds to the anus, is situate near the middle 

 of the inferior border. To this genus belongs an enormous 



shell of the Indian Ocean, known 

 under the vulgar name of the 

 Holy-water Pot ; it adheres to 

 the rocks by its byssus, which is 

 so large sometimes that an axe is 

 required to cut it, and the shell 

 itself occasionally weighs over 

 three hundred pouuds. 



28. The Chama have an ir- 

 regular, equivalve shell, which is 

 usually lamellar and rough ; ihey 

 live attached to rocks, coral, &c., 

 like oysters, and they have a 

 small foot bent like that of man 

 {fig, 102). 



29. ThelsocARDLE (^0^.104), 

 on the contrary, have a free, 

 regular, convex shell with spirally 

 curved summits, divided anterior- 

 ly. A large, smooth, red species, 

 the Chama cor, inhabits the 

 Mediterranean. 



Fig. 104. ISOCARDIA. 



27. What are the characters of the genus Trida'cna ? 



28. How is the genus Chama distinguished ? (Chama,— pronounced 

 fca'wo,— fiom tlie Greek, chad, I gape.) _ 



29. What are the characters of the genus Isoca'rdia ? (Isocardia, from 

 the Greek, \RUi>, iike, and kardia, a heart,— heart-shape.) 



