BIVALVIA. 249 



the Stevenston shell to M. subtruncata, but Mr. Smith tells me it does not belong to 

 that existing British species. 



I have, therefore, proposed the above name for the Crag fossil, not only from the 

 state of uncertainty respecting M. striata, but also from the consideration of the name 

 being employed for another species. 



11. Mactra constricta, S. Wood. Tab. XXIV, fig. 6, a — c. 



Spec. Char. Testa oblong o-ovatd, ineequilaterali, crassd, laevigata vel tenuissime striata; 

 antice longiore, rotundatd, postice angulatd ; margine ventrali constricto ; deniibus laterali- 

 bus perpendiculariter striatis. 



Shell ovately oblong, inequilateral, thick, smooth or finely striatedcon centrically ; 

 anterior side the longer, and rounded ; posterior angulated ; ventral margin contracted ; 

 lateral teeth rough or perpendicularly striated. 



Length, \\ inch. Height, finch. 



Localitg. Red Crag, Sutton. 



This is a common shell, and very abundant in some localities. In its adult state 

 it assumes a form somewhat approaching M. deaurata, the ventral margin on the 

 siphonal side, after a certain period of growth, becomes contracted or constricted, and 

 the dorsal margin then slopes suddenly, giving it an angular form. In the young- 

 shell, as indicated by the lines of growth, it has the oval shape of ovalis, and I con- 

 sidered it as a variety only of that species ; but in many of the species of this genus it 

 is very difficult to determine to which the immature shell belongs, and I have now 

 reason to believe this to be entitled to specific distinction. Mactra Caspia, Eichwald 

 (a Tertiary fossil), ' Fauna Caspio-Caucasia, p. 261, t. 38, figs. 21, 22, has some resem- 

 blance to our shell ; but it appears to be more ecpailateral, and too regular in form, 

 without the contraction or distortion of the Crag specimens. 



I have been unable to obtain any information respecting the specimen figured by 

 Woodward, called M. triangularis, ' Geol. of Norf.,' t. 2, fig. 9, though most obligingly 

 aided in the search by my collecting friends in Norwich. 



12. Mactra deaurata, Turton. Tab. XXIV, fig. 7, a, b. 



Mactra deaurata. Turt. Brit. Biv., p. 71, t. 5, fig. 8, 1822. 



— denticulata. W. Wood. Ind. Test. Supp. Mactra, pi. 1, fig. 9. 



— modicella? Conrad. Sillim. Journ., vol. xxiii, p. 340. 



Id. Foss. Med. Tert., p. 25, pi. 13, fig. 3, 1838. 



— jnequilatera. Nyst. Conch. Foss. Belg., p. 79, pi. 1 1, fig. 8, 1844. 



33 



