150 OYSTERS, AND ALL ABOUT THEM. 



two cardinal teeth, as in Placuna and Placenta, and is not 

 applicable to the present species. 



Family II. OSTREID^E, Broderip. 



BODY round : mantle having rather thick edges in 

 front : cirri short : gills simple. There is no foot, or 

 muscle for external 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 irregular shape, and inclined to be wedge-like, inequi- 

 valve : hinge toothless, but having its margins sometimes 

 notched : cartilage internal, short and curved, placed hori- 

 zontally 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 attachment, and 

 in the shells being either free or adhering to other sub- 

 stances by the lower valve, which is invariably larger and 

 deeper than the other. 



Genus OSTREA, Linne. PL 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 numer- 

 ous, and many of them are only distinguishable by very 

 slight characters. Almost every sea appears to have 



