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fectly silicized ; and, in the transparent parts, the angulated tent-like 
characters are beautifully shown in white and opaque markings. 
In the Essex bank I met with a fine specimen, which exactly agrees 
with V. Scotica, described by Dr. Maton and Mr. Racket, in the Lin- 
nean Transactions, Vol. vm. p. 81, t. 2, f. 3, except being double 
the size of the recent shell. Like that shell, it is thick, subcordated, 
subcompressed, with many regular, parallel, transverse ridges : umbo 
reclined: cordiform depression lanceolate. Inside glossy; teeth 
strong, oblique : margin plain. Length full an inch ; breadth nearly 
an inch and a quarter. 
Among the fossil shells which I obtained from the collection of Mr. 
Strange, are specimens which are marked Maryland; and which 
appear to agree very closely in their characters with V. Mercenaria. 
At the sale of Dr. Menish’s collection, I purchased a very large 
shell of this genus, measuring four inches and a half in length, and 
four inches three quarters in width. Its approximation, in form, to 
the preceding shells, but having no cordiform depression, and a plain 
margin, are circumstances which seem to allow of its being considered, 
at least, as very like to V- islandica. I am unacquainted with where 
it was obtained. 
I was favoured by Captain Gardiner, from the same collection, with 
a shell exactly agreeing, in its specific characters, with the preceding, 
but being of rather a less size. This shell is one of the Elackdown 
fossils : it is nearly perfect, and, although its outer surface has suffered 
some little decomposition, by which it is rendered of a dead white, 
yet the whole substance of the shell being now a clear calcedonic 
substance, the thickest part of the shell is in some degree translucent. 
The lateral teeth do not, I think, exist with sufficient distinctness 
in either of these shells, to warrant their removal to the genus Cyclas. 
CXYI. C'ytherea. An equivalved and rather inequilateral bivalve. 
The hinge with two or three approximated teeth, converging at the 
