BIVALVIA. 259 



Generic Characier. 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." — Clark. 



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 {Osteodesmida) 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 very considerable depth. Dead shells have been found 

 as deep as 110 fathoms. 



1. Thracia pubescens, Pidteney. Tab. XXVI, fig. 1, a — d. 



Mya pubescens. Pult. In Hutchins' Dorset., p. 27. 



— — Turt. Conch. Diet., p. 99, fig. 35, 1816. 



— DECLivis. Donov. Brit. Shells, vol. iii, pi. 82, 1801. 

 Anatina myalis. Des/t. 2d ed. Lamk., vi, p. 80, 1835. 



— — Crouch. Int. to Lamk. Conch., p. 7, pi. 4, fig. 1, a, b, 1827. 



— PUBESCENS. Ttirt. Brit. Biv., p. 45, 1822. 

 Ampiiidesma pubescens. Flem. Brit. An., p. 431, 1828. 

 Thracia pubescens. Kiener. Coq. viv. Thracia, p. 5, pi. 2, fig. 2. 



— — Besh. Exp. Moree. Zool., p. 87, pi. 18, fig. 1. 



— — Couth. Bost. Journ. Nat. Hist., vol. ii, p. 135, 1839. 



— — J. Soiv. Min. Conch., t. 631, fig. 1. 1844. 



— — Forbes. Jilgean Invert., p. 182. 



_ _ Forb. and Hani. Hist. Brit. Moll., p. 226, pi. 10, figs. 2, 3. 



— MoNTAGUi. Leach, MSS., 1818. Ann. and Mag. Nat. Hist., vol. xx, p. 272, 1847. 

 Llgula pubescens. Mont. Test. Brit. Supp., p. 23, 1808. 



