144 
OSTREA dorsata. 
TAB. CCCCLXXXIX — figs. 1 and 2. 
Spec. Char. Convex, subimbricated ; upper valve 
marked with numerous, longitudinal, branched 
striae ; inner margin toothed. 
Syn. Ostrea dorsata, Deshayes. 
The convexity of the upper valve of this Oyster appears 
to have given rise to the name dorsata ; but the form 
varies much, according* to the form of the substance to 
which it is attached : the numerous longitudinal im- 
pressed striae are its peculiar character, and we know of 
no other fossil Oyster so marked : we have seen a very 
flat recent one, of a purple brown colour, attached by a 
large surface, with similar striae. 
Fig. 1 represents a single valve from Hordwell: it is 
the only one that has fallen under our observation. 
Fig. 2 is an individual from France : it has a ridge 
along it produced by its having been attached to the 
stem of an Encrinus, and was sent us with the name of 
dorsata by Mons. Deshayes. 
OSTREA semiplana. 
TAB. CCCCLXXXIX.— fig. 3. 
Spec. Char. Oval, depressed, largely undulated ; 
in the middle flat • attachment small. 
Syn. Ostrea, No. 74, Mantell , Geol. Suss.p. 207. 
t. 25. f. 4. 
One of those Oysters that prefer thin cylindrical subjects 
for their support, and from which they spread out like 
leaves : it approaches the plicated division of the Genus 
by the largeness of its undulations ; but they are not an- 
gular, and the edge is not deeply sinuated by them. 
Although not abundant, this seems to be a generally 
diffused fossil of the upper Chalk; Mr. Mantell obtained 
it near Lewes. Our specimens are from Norfolk ; the 
larger, from a chalk-pit close to Norwich belonging to 
Mr. Barnes, was found at the depth of forty feet. 
