BIVALVIA. 167 



Shell triangularly orbicular, depressed, with 20 to 22 convex, close set nodulous 

 ribs ; concentrically sulcated ; umbones moderately elevated. 



Diameter, \ an inch. 



Localities. Cor. Crag, Sutton, Sudbourn, and Gedgrave. 

 Red Crag, Sutton, Walton-on-the-Naze. 



This species is very abundant in the Coralline Crag at Gedgrave and Sudbourn, 

 and it is by no means scarce in some parts of the Red Crag. The most distinguishing 

 character in this shell is the nodulous form of the ribs, which are placed near together, 

 separated only by a narrow line. The outline of this shell is in general pretty regular, 

 being of a roundly trigonal form, with the umbo turning slightly towards the anterior ; 

 occasionally the umbo is more elevated, giving it then a thicker hinge; a small smooth 

 space or lunule is visible, when perfect, on the anterior side, and the elevated ribs 

 produce a deeply crenulated margin. 



3. CARDITA ORBICULARIS. Leathes* MS. Tab. XV, fig. 4. 



^^ \ 



VENERICARDIA OBBICULARIS. J. Sowerby. Min. Conch., t. 490, fig. 3, 1825. 



, Potiez et Mich. Cat. des Moll, de Douai, p. 165, No. 17, 1844. 



CARDITA Nyst. Coq. FOBS, de Belg., p. 214, pi. xvi, fig. 10 a, b, d, 1844. 



TUBERCULATA. Gold/. Pet. Germ., vol. ii, p. 188, t. 134, fig. 3 a, b. 



Spec. Char. Testa orbiculari, tumidd, convexd, crassd; costis 16 18 distantibus ; 

 concentrice sulcatis. 



Shell orbicular, convex, tumid, and thick; 16 to 18 convex ribs, with a flat space 

 between them ; concentrically sulcated. 



Diameter, \ an inch. 



Localities. Cor. Crag, Sutton. 



Red Crag, Sutton, Newbourn. 



This species is also abundant both in the Coralline and in parts of the Red Crag, 

 but I have not seen it from Walton-on-the-Naze. The shell is more orbicular than 

 the preceding, and it is also more tumid ; the costae are different, being narrower and 

 standing apart, having a flat space between them, equal to the breadth of the ribs ; 

 the costae are ornamented with nodules like those of scalaris, but are fewer in number; 

 the interstices show the obtuse lines of increase, but they are not elevated like those 

 upon the ribs, and the umbo is rather more curved and less prominent. 



4. CARDITA CHAM^FORMIS, Leathes MS. Tab. XV, fig. 3 a b. 



VENERICARDIA CHAJLEFORMIS. J. Sow. Min. Conch., t. 490, fig. 1, 1825. 



Potiez et Mick. Cat. des Moll, de Douai, p. 162, No. 3, 1844. 

 CARDITA CHAM^EFORMIS. Gold/. Pet. Germ., vol. ii, p. 189, t. 134. fig. 4 a, b. 



Nyst. Coq. Foss. de Belg., p. 211, pi. xvi, fig. 7 a, 6, d t 1844. 



22 



