60 



PLACUNA. ANOMIA. 



either si??i2)le, or tvared, iheoiheY folded. O.edn- 

 lis, common oyster, belongs to the first division. 



" O. folium is of the second ; a curious species, 

 from the Indian and American seas : the shell is 

 fixed to wood and to the roots of trees on the sea- 

 shores. 



^' Vulsella is a foreign genus, from the Indian 

 and other seas. 



^' Placiina does not adhere to any marine sub- 

 stance. The valves are flat, thin, and trans- 

 parent ; the very small space between them shows 

 that the animal must be extremely flattened : 

 there are two singular ribs at the hinge in the 

 form of a V. 



" P. placenta^ Chinese window-glass, is so 

 transparent when young, that it serves instead 

 of that material in China. 



^' Anomia. The shells of this genus are fixed, 

 like the oyster, to marine bodies. They live and 

 perish on the spot where they are at first pro- 

 duced. I have noticed the muscle by which they 

 attach themselves. Lamarck informs us that a 

 hard, small operculum is to be seen at the ex- 

 tremity of this muscle, and fills up the hole in 

 the flat valve when the muscle is contracted. 

 (Plate 5.) 



" The family Puidista contains only a few 

 genera, which will be quite uninteresting to you 

 at present. 



