56 MOLLUSCA FROM THE CRAG. 
Animal orbicular, or of the form of the shell and capable of being entirely covered 
when the valves are closed, mantle with its margins simple and disconnected, some- 
what enlarged in the anal regions; a large semilunar shaped foot with undulating 
edges, permitting an expansion into a subdiscoidal form. No byssus.. 
This is purely a marine genus, inhabiting waters of various depths, with an 
extensive geographical range ; though it does not as yet appear to have been found in 
any of the very cold regions of the globe. In a recent state the shells are generally 
covered with a velvety epidermis, except about the umbones, where it is often 
worn off. 
The species are not numerous, either recent or in a fossil state, but appear to 
have long been inhabitants of this planet, two or three are described by Colonel 
Portlock, from the Silurian Rocks of Tyrone, and others have been found in the 
Oolites and Green sand, but the shells are not of any magnitude, until the Tertiary 
Periods, in which as individuals they are largely developed, both in Europe and in the 
Upper Tertiaries of America. 4 
This is a well-marked genus, and not likely to be confounded with any other, 
except Limopsis, from which, however, it may be distinguished by the lgamental 
area being simple, or only marked with angular or diverging lines, while in that shell 
the cartilage is more distinctly separated from the ligament, and placed in a triangular 
fossette immediately beneath the beaks. 
The ligament in this genus occupies the entire space between the umbo and the 
hinge margin, not equally spread over the surface, but placed in diagonal, or rather in 
lines diverging from the beak towards the lateral margins, by which a deep impress or 
furrow is formed and left upon that part of the shell. 
1. PECTUNCULUS GLYCIMERIS, Linneus. Tab. IX, fig. 1, a—A. 
Bonanni. Recr. Ment. et Ocul., fig. 61, 1684. 
Cnama Gtycrverts Betton. List. Hist. Conch., lib. iii, pars 11, fig. 82, and fig. 80? 
1687. 
PrcTuncuLus rossitts. Dale. Hist. and Antiq. of Harw., p. 291, t. xi, fig. 3, 1730. 
Arca Gtycimeris. Linn. Syst. Nat., ed. 12, p. 1143, No. 181, 1767. 
— — Poli. Test. Sicil., vol. ii, p. 144, t. 26, fig. 1; t. 25, fig. 19, 1795. 
— — Don. Brit. Shells, vol. ii, pl. 37, fig. 2, 1800. 
—_ _ Mat. and Rack. Linn. Trans., vol. viii, p. 94, t. 3, fig. 3, 1807. 
— — W. Wood. Ind. Test., p. 46, pl. 10, fig. 36, 1825. 
— — Mawe. Lin. Syst. Conch., pl. 13, fig. 7, 1823. 
-- — Burrow. Elem. of Conch., p. 143, pl. 8, fig. 7. 
— prnosa. Linn. Syst. Nat., ed. 12, No. 182, p. 1143, 1767. 
— — Poli. Test. Sicil., vol. ii, p. 138, t. 26, fig. 2—4, 1795. 
— — Broc. Conch. Foss. Subap., p. 487, 1814. 
— — Mat. and Rack. Linn. Trans., vol. viii, p. 94, t. 3, fig. 4, 1807. 
— -— W. Wood. Ind. Test., p. 46, pl. 10, fig. 37, 1825. 
Arca scripra. Born. Mus. Cees. Vin., p. 93, t. vi, fig. 1, 1780. 
— unpata. Chem. Conch. Cab., vol. vii, p. 224, pl. 57, fig. 560, 1784. 
