BIVALVIA. 247 



Locality. ? Cor. Crag, Sutton. 



Red Crag, Bawdsey, Sutton, Woodbridge. 



Mam. Crag, Bramerton, Chillesford. 



Clyde Beds. Recent, Finmark, Britain. 



The name of ovalis is selected in preference to that of dubia, as being best suited 

 to the character of the species, both having been proposed at the same time. 



A considerable range in variation is observable in the numerous specimens found 

 in the Red Crag, some of which very closely approach M. solida, so much so that I 

 was induced to consider it in my ' Catalogue ' only as a variety, more especially as in 

 this, when perfect, the dorsal portion is marked with ridges such as are supposed to be 

 a good distinguishing character for that species. The principal difference appears to 

 be a greater solidity of substance in the one so named, and the other is of course 

 thinner. Our Red Crag fossil has the exterior covered with very fine, close-set, and 

 rather irregular striae, while upon the upper or dorsal portion it is more strongly ridged 

 than is usual in the recent shell. The variety M. dubia is less elliptical, with the dorsal 

 portion ridged, connecting it in that character with M. solida, from which it differs only 

 in being rather more oval and thinner. 



In the Red Crag this is one of the most abundant shells, and may be obtained 

 in most localities ; but I have never found it at Walton-on-the-Naze. 



9. Mactra subtruncata, Da Costa. Tab. XXIV, fig. 3, a, b. 



Trigonella subtruncata. Da Costa. Brit. Conch., p. 198, 1778. 



Mactra subtruncata. Mont. Test. Brit., p. 93, and Sup., p. 37, pi. 27, fig. 1. 



_ _ Loven. Ind. Moll. Scand., p. 45, 1846. 



_ _ Forb. and Hani. Hist. Brit. Moll., vol. i, p. 358, pi. 21, fig. 8; 



pi. 22, fig. 2 ; and pi. L, fig. 3, 1848. 



_ — ? Mldd. Malac. Rossic, p. 581, t. 18, figs. 11— 13. 



— — Sismonda. Syn. Meth. Anim. Invert. Ped. Foss., p. 22, 1847. 



— cuneata. J. Sow. Min. Conch., t. 160, fig. 7, 1817. 



_ _ Woodw. Geol. of Norf., p. 43, t. 2, fig. 10, 1833. 



— Euxinica? Knjnichi. Bull, des Not. de Moscou, No. 11, p. 63, 1837. 



— limbata. Menke. Fide Loven. 



Spisula subtruncata. Gray. Mag. Nat. Hist., new series, p. 374, 1837. 



Spec. Char. Testa ovato-triangtdari, vel cuneiformi, inaquilaterali, crassd ; antice 

 breviore subtruncata ; postice producta, angulatd; dentibus later alibus perpendicidariter 

 striatis ; margine ventrali convexiusculo. 



Shell ovately triangular or obtusely wedge-shaped, inequilateral, thick ; anterior 

 side the shorter, and roundedly truncate ; posterior side produced, and angulated ; 

 beaks slightly prominent ; lateral teeth perpendicularly striated ; ventral margin 

 slightly convex. 



