66 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. 



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 ligamental 

 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, Linn&us. Tab. IX, fig. 1, a h. 

 Bonanni. Recr. Ment. et Ocul., fig. 61, 1684. 

 CHAMA GLYCIMERIS BELLONII. List. Hist. Conch., lib. iii, pars 11, fig. 82, and fig. 80? 



1687. 



PECTUNCULUS FOSSILIS. Dale. Hist, and Antiq. of Harw., p. 291, t. xi, fig. 3, 1730. 

 ARCA GLYCIMERIS. 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, pi. 37, fig. 2, 1800. 



Mat. and Rack. Linn. Trans., vol. viii, p. 94, t. 3, fig. 3, 1807. 



W. Wood. Ind. Test., p. 46, pi. 10, fig. 36, 1825. 



Mawe. Lin. Syst. Conch., pi. 13, fig. 7, 1823. 



Burrow. Elem. of Conch., p. 143, pi. 8, fig. 7. 



PILOSA. Linn. Syst. Nat., ed. 12, No. 182, p. 1143, 1767. 



Poli. Test. Sicil., vol. ii, p. 138, t. 26, fig. 24, 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, pi. 10, fig. 37, 1825. 

 ARCA SCRIPTA. Born. Mus. Goes. Vin., p. 93, t. vi, fig. 1, 1780. 



UNDATA. Chem. Conch. Cab., vol. vii, p. 224, pi. 57, fig. 560, 1784. 



