BIVALVIA. 243 



Spec. Char. Testa ovatd, transversa, suhtrigond, subaqiiUaterali, Icevi, tenui ; cardine 

 bidentatd, dentibus lateralibus acutls, elongatis ; shm palUari semkircidari. 



Shell ovate, transverse, subtrigonal, slightly inequilateral, smooth, and thin ; 

 hinge with two cardinal teeth in each valve, with sharp and compressed lateral teeth ; 

 impression of the mantle with a semicircular sinus. 



Length, 2j inches. Height, \\ inch. 



Locality. Cor. Crag, Sutton. 

 Red Crag, Sutton. 

 Mam. Crag, Bramerton. 

 Clyde Beds, Smith; Bracklesham {Bixon). 



Recent, Mediterranean and British Seas. 



A fragment only of this species is in my cabinet from the lower or CoralUne Crag, 

 and it is not by any means abundant in the succeeding or Red Crag period, but it has 

 been found plentifully in the Mammaliferous Crag, near Norwich, and in the Clyde 

 beds, and it has become a very common shell in our own seas. 



There is considerable variation among my specimens not only in proportionate 

 dimensions but also in the outward form. In fig. a the shell is almost as high as it is 

 long, with the dorsal edge rounded ; fig. b, is comparatively much longer, with the 

 hinder side angulated and somewhat pointed. The impressions of the adductors are 

 rather large, and the sinus of the mantle-mark extends inwards about ^ths of the length 

 of the shell. The lateral teeth are thin, double in the right valve, and the cardinal 

 A-formed tooth diverges nearly at a right angle. 



In the recent state it is littoral in its habits, and has a wide geographical range, 

 extending from the south coast of Norway {Forbes and Hanley) into the Mediterranean, 

 and it is mentioned by Philippi as having been met with by Ehrenberg in the Red Sea. 



3. Mactra arcuata, /. Sotverbg. Tab. XXIII, fig. a — c. 



Mactka arcuata. J. Sow. Min. Coucli., t. ICO, fig. 1, 1817. 



— — Woodw. Geol. of Norf., p. 43, 1833. 



— — Nyst. Coq. Foss. de Belg., p. 78, pi. 11, fig. 7, 1844. 



Spec. Char. Testa ovato-trigonula, inceqiiilaterali, Icevigatd, subcotnpressd, tenui ; antice 

 latiore et breviore, rotundatd ; margine dorsali arcuatd; cardine bidentato, dentibus 

 lateralibus perpendicidariter striatis ; sinu palliari Unguiformi. 



Shell ovate, or rather ovato-trigonal, inequilateral, smooth, slightly compressed, 

 thin; anterior side the broader, short, and rounded, dorsal margin curved; hinge with 

 two cardinal teeth, and striated lateral teeth ; pallial sinus tongue-shaped. 



Length, 3j inches. Height, 2|- inches. 



Locality. Coralline Crag, Sutton, Gedgrave. 



Red Crag, Walton Naze, Sutton, Bawdsey. 



