BIVALVES. — OSTREA. 63 



ophytes : they have the faculty of leaping out of the water, 

 and are enabled to effect a very rapid motion, by opening 

 and closing their valves. 



The third division of Ostreae consists of those which 

 in construction, substance, and colouring, are more nearly 

 allied to the common or eatable oyster. The species of 

 these divisions are of a much more irregular form than the 

 scallops, and are usually very rugged, unfinished looking 

 shells. The hammer oyster, O. malleus, is perhaps the 

 most remarkable of this class ; its form resembles that of 

 a long-headed hammer, or more properly a pick-axe : there 

 are two distinguishable varieties of it, viz. the white and 

 the black, both of which, when in fine preservation, 

 are considered rare and valuable. These shells are rough 

 and plaited on the outside, but their inside is smooth and 

 glossy, having a steel blue colour or metallic lustre difflised 

 over the surface. 



The hinge of some of the species, as the O. perna and O. 

 isognomon, has a pei-pendicular grooved line attached to 

 it. > Some, again, as the O. lima, &:c. gape at the hinge ; 

 others terminate in a long beak from the hinge upwards, 

 as the O. virginea. 



The species of the fourth division- are parasitical, and 

 some have the appearance of a dried leaf, as the O. folium, 

 &c. which often adhere to the roots and stumps of trees, 

 especially the mangrove, and are also found affixed to the 

 gorgoniae. 



The O. crista-galli and the other species of the third fami- 

 ly of the fourth division were, in the former editions of this 

 work, which closely adopted Gmelin's arrangement, classed 

 among the Mytili ; but, guided by the character of the shells, 

 we have removed them to this genus, to which they indis- 

 putably belong. These attach themselves to foreign bodies 

 by a formation of the shell itself, which has the appearance 

 of several distinct claws or hands. 



