BIVALVIA. 259 
Generic Character. Shell transversely ovate, inequivalve, often nearly equilateral ; 
tumid or compressed, generally thin, slightly gaping; surface smooth or minutely 
granulated. Hinge with an internal cartilage attached to a projecting callosity. Often 
a small cleft in the umbo, formed by the ligament. Impressions by the adductors 
unequal; mantle-mark deeply sinuated. 
“ Animal ovate; mantle closed, except for the passage of a compressed linguiform 
foot: siphons rather long, separated to their bases, and furnished with fimbriated 
orifices, which are often inflated into a globular form.” —C/arf. 
M. Deshayes having discovered a detached ossiculum in the hinge of one or more 
species possessing somewhat similar external characters, presumed it to exist in all, 
and proposed, in consequence, a family (Osteodesmide) founded upon this character, 
distinguishing the genera by the peculiar form of this “little bone,” and its position 
in the hinge, it being held between the cardinal callosities by a portion of the internal 
ligament, sometimes close to the anterior, at others on the opposite part of the 
cartilaginous area. 
The distinctions founded upon such characters are but doubtfully sufficient for 
generic separation, even where its position could be correctly determined. There is, 
however, a peculiarity in the calcareous callus of the hinge, or support for the 
ligament, in this genus, sufficient to prevent its being confounded with any other. 
The shells are generally thin, with a rugose or scabrous exterior. They are 
probably of great antiquity, as fossils of this form are found in the lower Oolites, and 
doubtfully so in the Carboniferous series. 
In the living state, the species frequent sandy or sandy-mud shores, and have a 
range from low-water mark to ae considerable depth. Dead shells have been found 
as deep as 110 fathoms. 
1. THRACIA PUBESCENS, Pulteney. Tab. XXVI, fig. 1, a—d. 
Mya pupescens. Pult. In Hutchins’ Dorset., p. 27. 
— — Turt. Conch. Dict., p. 99, fig. 35, 1816. 
— pecLivis. Donov. Brit. Shells, vol. iii, pl. 82, 1801. 
Anatina MyaLis. Desh. 2d ed. Lamk., vi, p. 80, 1835. 
— — Crouch. Int. to Lamk. Conch., p. 7, pl. 4, fig. 1, a, 6, 1827. 
— pusescens. Turt. Brit. Biv., p. 45, 1822. 
AMPHIDESMA PUBESCENS. Fem. Brit. An., p. 431, 1828. 
THRACIA PUBESCENS. tener. Coq. viv. Thracia, p. 5, pl. 2, fig. 2 
— — Desh. Exp. Moree. Zool., p. 87, pl. 18, fig. 1 
= — Couth. Bost. Journ. Nat. Hist., vol. ii, p. 135, 1839. 
= — J. Sow. Min. Conch., t. 631, fig. 1, 1844. 
— — Torbes. Mgean Invert., p. 182. 
— — Forb. and Hanl. Hist. Brit. Moll., p. 226, pl. 16, figs. 2, 3 
— Montacur. Leach, MSS., 1818. Ann. and Mag. Nat. Hist., vol. xx, p. 272, 1847. 
LucuLa puBEscens. Mont. Test. Brit. Supp., p. 23, 1808. 
