BIVALVIA. 91 



so much so as in L. lanceolata, and the muscular marks are then deeply impressed, 

 those produced by the adductors are of a subovate form, and are unequal in size, the 

 anterior one being the larger, and the sinus formed by the mantle extends about as far 

 inwardly as the posterior part of the ligamental pit, which is broad and contracted in 

 the centre. 



Our shell appears to correspond with the recent American species to which 

 it is here assigned, in all its characters, excepting size. Dr. Gould gives the dimen- 

 sions of his shell as 1^ inch in its largest diameter, which is considerably less than 

 the magnitude to which our fossil has attained. In the recent state it is generally 

 eroded at the beaks ; our fossil does not appear to have been at all acted upon at 

 that part, it is there quite perfect. 



A shell from the Antarctic Seas, N. Eiglitsii, Coutkouy, strongly resembles our 

 British fossil. 



3. LEDA SEMISTRIATA, S. Wood. Tab. X, fig. 10, a b. 



NUCULA SEMISTRIATA. S. Wood. Mag. Nat. Hist., New Series, vol. iv, p. 297, pi. 14, 



fig. 5, 1840. 



NITIDA. Nyst. Rech. Coq. Foss. d'Anv., p. 16, pi. 3, fig. 62, 1835, (not Brocchi.) 

 DEPRESSA. Nyst. Coq. Foss. de Belg., p. 220, pi. 15, fig. 7, 1844. 



Spec. Char. Testa transversd, ovato-ellipticd, subaquilaterd, compressd, tenuissimi, 

 fragili ; antice rotundato-ovatd et Icevigatd, postice subrostratd et transversim striatd ; 

 natibus approximatis, margine integerrimo. 



Shell transverse, elliptically ovate, subequilateral, compressed, thin, and fragile ; 

 anterior side roundly ovate and smooth ; posterior subrostrated and transversely 

 striated ; beaks approximate, margin quite smooth. 



Longest diameter, 1 inch. 



Locality. Cor. Crag, Sutton and Ramsholt. 



This shell is abundant in the Coralline Crag at Sutton, but from its great fragility 

 specimens of the above dimensions are very rarely obtained, and if it be the same as 

 the Belgian fossil, which I presume is the case, M. Nyst speaks of it as being by no 

 means rare in the Campinian Beds. 



The striae or transverse ridges upon our shell are rounded and obtuse, not sharp 

 or imbricated ; they cover the posterior half of the exterior, extending from a little 

 beyond the centre or umbo to the extremity, but often become irregular and obsolete 

 upon the posterior slope, and the shell is there depressed and subsinuated with a very 

 slightly recurved and somewhat pointed termination at the extremity of the dorsal 

 edge. From the extreme thinness of the shell the transverse edges are often visible 

 in the interior ; a long line of sharp angularly formed and prominent teeth occupy 

 the hinge area, amounting in large specimens to as many as five and twenty on the 

 anterior, with twenty or more on the opposite side, while in young ones they are not 

 above half that number : they are separated by a rather large and obtusely angular 



