IilVALVIA. 15 



Diameter, £ of an inch. 



Locality. Ilampstcad (Edwards). 



Two or three specimens arc all that I have seen. The imbrications upon the upper 

 valve are large and elevated, presenting a very rough exterior, resembling the surface of 

 a blacksmith's file. The lower valve shows the round foramen to be on the left side of the 

 umbo, from which I presume the exposed surface to be the interior of that valve. 



We have thus in these older Tertiaries the prototypes of the striated and imbricated 

 recent British species, only in excess, the one more roughly imbricated, and the other less 

 coarsely striated. 



OSTREA. Z»»«.,1685. 



Generic Character. Shell attached by the larger or lower valve, generally thick and 

 strong, lamellated or foliated, variously shaped, irregular, inequivalved, inequilateral ; 

 upper or free valve flat or slightly concave ; under valve convex, sometimes strongly marked 

 with radiating, lamellated costre ; hinge without teeth ; connexus ligamentous lodged in an 

 elongated, triangular depression in each valve. Impression of the adductor muscle large, 

 subcentral, that formed by the mantle entire, generally indistinct and ill-defined. 



Animal with the mantle-margin double, or disunited ; its edges bordered by short, 

 tentacular fringes ; foot obsolete. Sexes distinct. 



The oyster fixes itself by the outside of the left valve, and as this is done generally upon 

 a horizontal support, the valves, from that position, are called upper and lower, and although 

 they are unsymmctrical and inequivalved, they are nevertheless bilateral, and have a right 

 and left valve like the Dimyaria. Oysters are generally gregarious animals, although some 

 species appear to be solitary. Osfrea folium, an Oriental species, secretes projecting 

 processes or fingers, which extend from the back of the lower valve, and by which 

 it clasps the roots and branches of trees which grow into the water, from which 

 habit it was called Dendostrea by Swainson. This, of course, is done when the 

 animal is young, or only so long as the edge of the mantle can be extended to 

 the extremity of the processes, after which they cannot be prolonged. Some oysters 

 are peculiarly prone to secrete a large quantity of lime, particularly where that material is 

 abundant ; and a fossil oyster from the banks of the Tagus has been found with its lower 

 valve two feet in its longest diameter, and of a proportional thickness. The oyster, in 

 general, is adherent in the younger state, but when it has grown large and heavy it ceases 

 to increase the attachment, and enlarges the shell, like a free Mollusc. Some species adhere- 

 only by a very small portion of the shell, while others are attached by nearly the whole of 

 the outer surface of the lower valve ; this character is, however, variable, even amongst 

 individuals of the same species. The genus inhabits salt water, although the common 

 edible oyster will live in rivers in England where the water at low tide is nearly fresh. 



The age of the oyster is probably various in different species ; 0. edtdis is said to live 



