BIVALVIA. 115 



teeth, it then slopes to the sides which are both rounded, and the ventral margin is 

 also slightly convex, differing thus a little in not presenting quite such a quadrate 

 form as the recent shell ; perhaps a larger number of individuals both recent and fossil 

 would present a greater resemblance. The beautiful sculpture which ornaments the 

 recent shell, is replaced in the fossil by a granulated surface, the effect of probably 

 unequal erosion, and the semipellucid appearance is changed into an opaque one from 

 the loss of its animal matter. Two ovate rather deeply impressed muscle marks are 

 distinctly visible in my specimen, which measures barely a quarter of an inch in 

 length, and a little less in height. 



2. LEPTON DELTOIDEUM, S. Wood. Tab. XI, fig. 9, a d. 

 KELLIA DELTOIDEA. S. Wood. Catalogue, 1840. 



Spec. Char. Testa subtriangulatd, vel deltoided, tequilaterali, tumidd, politd, fragili ; 

 utrinque rotundatd, margine ventrali recta ; dentibus later alibus approximatis. 



Shell triangular or deltoidal, equilateral, tumid, glossy, and fragile ; anterior and 

 posterior sides rounded with the ventral margin straight, lateral teeth approximate. 



Length, ^ an inch. Height, T \ths. 



Locality. Cor. Crag, Sutton, Ramsholt. 

 Red Crag, Sutton. 



This delicate and very elegant species is not particularly scarce in the Coralline 

 Crag, where I have procured more than a couple of dozen specimens, 'and notwith- 

 standing its extreme fragility, its presence in the Red Crag is also undoubted, two 

 specimens belonging to that Formation are in my Cabinet, where they have been for 

 many years, but their exact locality is uncertain, as the label has been unfortunately 

 lost. We may fairly presume it to have prolonged its existence into the Period of the 

 latter Deposit, as such delicate shells could only under very favorable protection 

 have survived, being washed from an Older into a Newer Formation, and I give them 

 without hesitation as natives of the Seas of that part of the world in both Periods. 



I am not acquainted with any described species to which this can with certainty be 

 assigned. Bornia corbuloides, Phil., En. Moll. Sic., vol. i, p. 14, 1. 1, fig. 15, somewhat 

 resembles it in outline, but the difference as given by the description appears 

 to present characters sufficient to keep them distinct, being recorded to have 

 its margin crenulated on both sides, which our shell certainly has not. 



Lepton fabagetta, Conrad, a very indifferent figure of which is given by Dekay, 

 in the ' Nat. Hist, of New York,' a little resembles our shell in outline, and a fossil 

 species by Conrad, Lepton mactroides, from the Upper Tertiaries of America, present 

 general or generic resemblance, but the specimens must be examined for correct 

 determination, and I have not been able to see any of the three species above referred 

 to : our fossil must, therefore, for the present, remain with the name originally imposed 



