PLACUNA. 253 



and flowers of tropical regions^ extend occasionally to the 

 unassuming Oyster. Many of the species are beautifully 

 shaded^ and the shells of such as inhabit the Red Sea are 

 frequently varied \Yith the vivid colours of the rainbow. 



The next genus^ Placima, is easily recognized by the 

 thinness and flatness of the valves, and peculiar construction 

 of the hinge, which consists of two distinct ribs or ridges, 

 on one valve only, diverging upwards ; and to the outer 

 edge of these is affixed the ligament. 



Specimens of PlacuMuiomia vrere found attached to dead 

 bivalve shells and corals, at the depth of eleven fathoms, in 

 the Gulf of Dulce, province of Costa Ptica, Central America, 

 by Mr. Cuming, who had spent many years in diving and 

 dredging for this remarkable genus, which, as the name 

 denotes, is intermediate between the Placiuui and Anomia. 

 Like the former, the hinge consists of two divaricate ribs, 

 bearing the ligament; and, like the latter, there is an 

 opening in the lower valve for the passage of a bony tendon, 

 by which the mollusk attaches himself to liis watery location. 

 This tendon, however, is rather a singular modification 

 of that which pertains to the Anomia : instead of directly 

 perforating the lower valve, it is first introduced, like a flat 

 plate, between tlie laminse of which the valve is composed, 



