OSTREA. 37 



lies in having two cardinal teeth, as in Placuna and Pla- 

 centa, and is not applicable to the present species. 



Family II. OSTRElD^l, Broderip. 



BODY round : mantle having rather thick edges in front : 

 cirri short : gills simple. There is no foot, or muscle for ex- 

 ternal attachment. The animal is fixed in the earlier stage of 

 its growth, and sometimes in its adult state, by the lower or 

 more convex valve of its shell. 



SHELL circular, longitudinally oval or oblong, or of an irre- 

 gular shape, and inclined to be wedge-like, inequivalve : hinge 

 toothless, but having its margins sometimes notched : cartilage 

 internal, short and curved, placed horizontally on the hinge- 

 line. 



Some genera are exotic, and others are extinct or 

 known only as fossil. We have but the typical genus. 

 The Oyster family differs from that of Anomia in the 

 gills being simple, in having no foot or plug of attach- 

 ment, and in the shells being either free or adhering to 

 other substances by the lower valve, which is invariably 

 larger and deeper than the other. 



Genus OS'TREA *, Linne. PI. I. f. 5. 



BODY compressed. 



SHELL composed of numerous imbricated or tile-like plates, 

 which overlap one another in succession: beaks disunited: 

 cartilage strengthened by a ligament on each side of it. 



The so-called species of Ostrea are exceedingly nume- 

 rous, and many of them are only distinguishable by very 

 slight characters. Almost every sea appears to have 

 several species or varieties. Their general form is very 

 inconstant and often irregular. It is more than pro- 

 bable that when a sufficiently extensive series from each 

 * Oyster. 



