188 



silicized ; and, in the transparent parts, the angulated tent-like charac- 

 ters 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, sub- 

 compressed, with many regular, parallel, transverse ridges : umbo re- 

 clined: 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 F, islandica. I am unacquainted with where it was ob- 

 tained. 



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 Blackdown fos- 

 sils : 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. 



CXVI. Cytherea. An equivalved and rather inequilateral bivalve. 

 The hinge with two or three approximated teeth, converging at the base; 



