B1VALVIA. 123 



species in the British Museum, it presents the following differences : — N. 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 Heme 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 N. 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 Heme 

 Bay. This I at first thought to be probably the perfect condition of N. 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 sextans, the outline of it forming somewhat 

 irregularly the sixth part of the circle. Fig. 8 b, of the same plate is the likeness of a shell 

 in Jermyn Street, from Heme 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, T 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 

 pardonable 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 N. laevigata of the Crag, and I have given to it the name iV. prailcemgata, 

 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 JY.proava in being thinner, and in 

 having the anal region shorter than in that species ; and it differs from N. laevigata 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. 



17 



