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. 
Keira pELToIDEA. S. Wood. Catalogue, 1840. 
Spec. Char. Testa subtriangulata, vel deltoided, equilaterali, tumidd, polité, fragili ; 
utrinque rotundatd, margine ventral recta ; dentibus lateralibus approximatis. 
Shell triangular or deltoidal, equilateral, tumid, glossy, and fragile; anterior and 
posterior sides rounded with the ventral margin straight, lateral teeth approximate. 
Length, } aninch. Height, =®,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. 
Iam not acquainted with any described species to which this can with certainty be 
assigned. Bornia corbuloides, Phil., En. Moll. Sic., vol. i, p. 14, t. 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 fabagella, 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 
