BIVALVIA. 123 
species in the British Museum, it presents the following differences :—JV. fragilis has 
the basal margin on the anal side more pointed, with the centre of the anal region more 
elevated and distinct, and the pedilateral margin is not quite so much rounded: there 
appears also to be a difference in the number of denticles, but a larger number of specimens 
will be required to determine its correct position. 
Tab. XX, fig. 9, is a shell recently obtained by Mr. Gibbs from the Basement-bed of 
the London Clay at Herne Bay, and the officers of the Museum-in Jermyn Street, to 
which it belongs, have kindly permitted me to have it figured. I am unable to refer it to 
any known species, and propose to give to it the name of JV. striatella. It appears to be 
specially distinguished by an unusually prominent anal region ; it is finely rayed, and the 
margins are denticulated. 
Tab. XX, fig. 8 a, represents a specimen also from the Thanet Sands at Herne 
Bay. This I at first thought to be probably the perfect condition of WV. cardioides, 
which the young or unmutilated portion of that shell somewhat resembles; but upon 
close comparison the two specimens do not satisfactorily accord. Mr. Edwards has 
given to this specimen the MS. name of seatans, the outline of it forming somewhat 
irregularly the sixth part of the circle. Fig. 8 4, of the same plate is the likeness of a shell 
in Jermyn Street, from Herne Bay, which may probably belong to the same species, coming, 
as it does, from the same bed ; but I have had it represented in consequence of a difference 
in outline, it being less triangular than fig. 8 a, the dorsal margin on the pedal side being 
more convex, and the pedilateral margin less pointed. ‘The anal region (which, I think, in 
general affords a good auxiliary character) is, unfortunately, in this specimen broken. 
The recent discovery of these last-noticed specimens has caused considerable delay in 
the preparation of my work, but they appear of so much interest and of so much importance 
towards a history of the contents of our Eocene deposits, that I thought it would not be 
pardonakle to pass them over, and now, at the eleventh hour, another shell has come into 
the possession of Mr. Edwards, which appears to me to be also worthy of especial notice, 
and I have had it figured. 
Tab. XIX, fig. 21.—This last specimen is a very elegant shell ; it approaches in form 
very closely to WV. /evigata of the Crag, and I have given to it the name WV. prelevigata, 
in consequence of its very near relationship. It is excessively thin, quite smooth externally, 
and it has a margin free from crenulations ; it differs from JV. proava in being thimer, and in 
having the anal region shorter than in that species; and it differs from JV. /evigata in having 
a depression or shallow sulcus on the dorsal region beneath the dental edge; the curva- 
ture of the ventral margin, although nearly as great as in the Crag shell, is not quite so 
regular, and the pedilateral margin being a little broader in our present specimen, will 
distinguish it. The interior I have not been able to examine. The shell is too thin and 
too firmly imbedded in the matrix to permit of removal. 
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