BIVALVIA. 77 
margin being nearly straight, or with a very little inflection. A full grown specimen 
found in the Red Crag (fig. 1, a—4), is very regular in form, and all its ornamental 
strie beautifully preserved: the rays or costulated striz are close and numerous 
upon the posterior half of the outer side of the umbonal ridge, while they are 
larger and more distant upon the anterior half; and within the prominent ridge, 
that slopes from the umbo to the posterior ventral margin, the rays are large 
and few, amounting to about four or five, and are placed in pairs; they project 
beyond the posterior margin, giving it a jagged or indented edge. A long line 
of teeth or crenulations occupy the edge of the hinge margin, they are numerous 
and vertical on the anterior or shorter side, fewer or more distant, and strongly 
inclining on the posterior side. The ligamental area is large and concave removing 
the umbones far apart ; this space is ornamented with deep and angular lines diverg- 
ing from the umbo, in some they are few and deeply impressed, while in others they 
are morenumerous. One specimen in my Cabinet, from the Coralline Crag at Gedgrave 
(fig. 1, ¢), precisely resembles the distorted specimens found occasionally upon our 
own Coast, in holes or crevices of rocks, and in which the regularity of form has 
been interrupted, and the surface much abraded by frequent movements in a con- 
fined position, thus producing so great an alteration in the exterior of the shells as to 
have induced some authors to consider them distinct. In some of these full grown 
and distorted specimens, the ventral margin is deeply indented or sinuated. They 
are said by British Conchologists to be regular in form when free, and only distorted 
when confined to the crevices of rocks. 
I have entered it among my synonyma upon the authority of Menke, who gives it 
as one of the existing species from the North Western Coast, New Holland. He has 
enumerated several other European shells from that part of the world. 
2. ARCA LACTEA, Linnaeus. Tab. X, fig. 2, a—d. 
Arca LacTEa. Linn. Syst. Nat., ed..12, No. 173, p. 1141, 1767. 
— — Da Costa. Brit. Conch., p. 171, pl. 11, fig. 5. 
— — Chem. Conch. Cab. t. vii, p. 200, t. 55, fig. 547, 1784. 
—_ — Don. Brit. Shells, vol. iv, pl. 135, 1803. 
_ —_ W. Wood. Ind. Test., p. 45, pl. 9, fig. 24, 1825. 
— —_ Brown. Illust. Conch. Gr. Brit., pl. 25, fig. 6, 1827. 
— — Mawe. Linn. Syst. Conch., pl. 13, fig. 4, 1823. 
— — Phil. En. Moll. Sic., vol. i, p. 57; vol. ii, p. 42. 
_ Forbes. Rept. on Aigean Invert., p. 181, 1843. 
— — Reeve. Conch. Icon. Arca, pl. 17, fig. 116. 
— — Dujard. Mem. Geol. Soe. de France, t. ii, pt. 2, p. 266, 1837. 
= — Hanley. Recent Shells, vol. i, p. 154, pl. 9, fig. 24. 
— moprotus. Poli. Test. Sic., vol. ii, p. 137, pl. 25, figs. 20, 21, 1795. 
= — Turt. ed. Linn., vol. iv, p. 251, 1806. 
— pERFORANS. Turt. Conch. Dict., p. 9, 1816. 
— _ Id. Brit. Biv., p. 169,.t. 13, figs. 2, 3, 1822. 
— Gaimarpil. Payr. Cat. Moll. Cors., p. 61, pl. 1, figs. 36—39, 1826. 
