BIVALVIA. 
— 
cr 
Diameter, } of an inch. 
Locality, Wampstead (Ldwards). 
Two or three specimens are 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. 7 
OSTREA. Znnn., 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 cost ; 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-maygin 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 unsymmetrical and ineqnivalved, 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. Ostrea 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; O. edulis is said to live 
