BIVALVIA. 197 



more essentially a species of an Arctic character. From the examination of a large 

 suite of fossil specimens obtained of different ages, I am convinced no specific sepa- 

 ration can be made between these fossils and the shells now found living in our own, 

 and in the Arctic seas ; the form is a character upon which no dependence can be 

 placed, as in some specimens the diameter in a longitudinal direction is considerably 

 greater than when measured from the umbo to the ventral margin, as may be seen by 

 the specimens that have been figured ; while in others it is nearly orbicular, with the 

 height equal or rather exceeding that of its length ; as a general rule it may perhaps 

 be said that the umbo of the Cor. Crag shell is thicker and more prominent than it is 

 in the living species, but even in this there is considerable variation amongst my 

 fossils, and in the shells of the more modern deposits this is less distinguishable. 



This shell is said to attain in the living state to the large dimensions of five and a 

 half inches in length, but I have never seen a Crag specimen of such magnitude ; mine 

 range from 4^ inches, down to very little more than ^th of an inch. 



2. CYPRINA RUSTICA, /. Sowerby. Tab. XVIII, fig. 1 a e. 



VENUS RUSTICA. J. Sow. Min. Conch., t. 196, 1818. 

 CYPRINA TUMIDA. Nyst. Rech. Coq. Foss. d'Anv., p. 9, pi. ii, fig. 32, 1835. 

 Potiez and Mich. Cat. de Moll, de Douai, t. ii, p. 221. 

 Nyst. Coq. Foss. de Belg., p. 148, pi. x, fig. 1 a, a, 5, b, and pi. viii, 



figs. 2, 3, and 4, 1844. 

 RUSTICA. S. Wood. Catalogue, 1840. 



LAJONKAIRII. Gold/. Pet. Germ., vol. ii, p. 237, t. 148, fig. 9 a c, 1841. 

 . DEFRANCII. Fan Beneden. Bull. Zool. de Guerin, 1835, p. 149, fide Nyst. 



Spec. Char. Testa tumidd, globoso-cordatd, valde in&guilaterali -, latere postico sub- 

 quadrato ; subtilissime punctato-striatd ; umbonibus approximatis, subincurvis ; lunuld cor- 

 datd, profundd. 



Shell tumid, globosely heartshaped, very inequilateral ; posterior side subquadrate ; 

 covered externally with fine, radiating, and punctated striae ; umbones approximate, 

 slightly incurved; lunule heartshaped and deep. 



Length, 2f inches ; height, 2 j inches ; depth, 2| inches. 



Localities. Cor. Crag, Sudbourn, Gedgrave, Ramsholt. 

 Red Crag, Sutton, Newbourn. 



This species is less abundant than the preceding one, although in the Coralline Crag 

 at Gedgrave it is not of rare occurrence ; from the Red Crag I have only been able to 

 obtain a couple of specimens. The Belgian shell, which I believe to be the same 

 species, is stated by M. Nyst to be very abundant in several localities. 



This being undoubtedly the Venus rustica of Sowerby, I have retained his name as 

 having a priority of date, and this author appears to have been the first to notice the 

 species. The right valve has two cardinal teeth,the anterior one is of a triangular form, the 

 posterior one is elongated and sharp, they fit into depressions of the same form between 



