GENUS XIV. OSTREA. 73 



larger than those on the other side ; the largest ear in one 

 valve is also generally of a different shape from the corre- 

 sponding one in the opposite valve : these constitute the second 

 division of Linnaeus. In both these divisions there are some 

 of the species that have the valves unequal, one being flat and 

 the other convex, as O. maxima, &c. ; but perhaps the greater 

 number of species have both valves equally convex, or nearly 

 so, as O. opercularis, sanguinolenta, &c. None of these are 

 fixed to the rocks or any other place, but swim about freely. 

 The shells of the third division have likewise ears on each side 

 the beaks, but they are very small ; the hinge is differently 

 constructed, and stands somewhat obliquely ; the valves when 

 shut together leave a small vacancy just below the ears, through 

 which apertvire the animal protrudes a foot a little similar to 

 that of Area Noce, by means of which it attaches itself to any 

 particular place, or removes itself from one place to another. 

 The fourth division contains not only those commonly called 

 Oysters, but also several others, the hinges of which are totally 

 different : in the common Oysters which have no ears the 

 valves are unequal, the upper one in general is flat, and the 

 lower convex ; but, as these shells are fixed to rocks or some 

 other substance, it oftentimes depends on the form of the sub- 

 stance to which they happen to be attached, as in some cases 

 the lower valve is flat and the upper convex. The hinge is 

 without teeth, but there is often a crenatvu'e on the margin 

 extending a little way on each side the hinge : in the upper 

 valve there is scarcely any hollow for receiving the cartilage ; 

 but in the lower one there is a considerable one, which in- 



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