BIVALVIA. 249 
the Stevenston shell to J/. 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 JZ. striata, but also from the consideration of the name 
being employed for another species. 
11. Macrra constricta, S. Wood. Tab. XXIV, fig. 6, a—e. 
Spec. Char. Testa oblongo-ovatd, inequilaterali, crassd, levigatd vel tenuissime striata; 
antice longiore, rotundatd, postice angulatd ; margine ventrali constricto ; dentibus 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, 3 inch. 
Locality. Red Crag, Sutton. 
This is a common shell, and very abundant in some localities. In its adult state 
it assumes a form somewhat approaching J. deawrata, 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 ova/is, 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. MJactra Caspia, Kichwald 
(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 equilateral, 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 MW. 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, 0b. 
MactTrRa DEAURATA. Turt. Brit. Biv., p. 71, t. 5, fig. 8, 1822. 
—  peENTICULATA. W. Wood. Ind. Test. Supp. Mactra, pl. 1, fig. 9. 
— MopiceLLa? Conrad. Sillim. Journ., vol. xxiii, p. 340. 
— — Id. Foss. Med. Tert., p. 25, pl. 13, fig. 3, 1838. 
— tneEquiLaTeRA. Nyst. Conch. Foss. Belg., p. 79, pl. 11, fig. 8, 1844. 
33 
