"In the first genus we find the great Tridacna gigas, the largest and heaviest shell yet known. It sometimes weighs five hundred pounds. The hinge has two teeth, the lunula is open, the valves equal, the ligament exterior.
"Mytilacea.
| Modiola | Mytilus. |
| Mytilus | Mytilus. |
| Pinna | Pinna. |
"The greater part of these genera attach themselves to marine substances by a byssus. The Modiola genus are rarely found fixed. The ligament internal, lodged in a marginal gutter. Beaks nearly lateral; hinge without teeth. The genus Pinna is unaltered. Small crustaceous bodies, resembling the crab, are sometimes found in the shells of the Pinna.
PINNA.
PERNA.
AVICULA.
"Malleacea.
| Crenatula | Rare and little known. | |
| Perna | Ostrea. | |
| Malleus | Ostrea, hammer-oyster. | |
| Avicula | Mytilus. | |
| Meleagrina | Mytilus. |
"The first genus is found in the seas of warm climates. The shells are thin and foliated. The hinge of Perna differs widely from that of the oyster. It is linear, formed of sulcated teeth. There is a sinus under the extremity of the hinge, for the passage of the byssus. Compare P. isognomon with the common oyster, and you will find few points of resemblance between them. ([Plate 5.]) Perna ephippium is also a curious species, very pearly within. The hammers are rugged and singular in form. They are all foreign, from the oriental seas.