BIVALVIA. 129 



Spec. Char. Testa minufd, transversa, ohliqud, ovatd, valde incequilaterali, comexd, 

 politd, tenui, fragili ; antice produdd, utrinque rotundatd ; costatd, costis acutis paucis ; 

 natibus prominulis ; dente cardinali unico. 



Shell small, transverse, oblique, ovate, very inequilateral, convex, glossy, thin and 

 fragile ; anterior much the larger, both sides rounded ; ornamented with a few small 

 sharp radiating ribs ; umbones rather prominent ; hinge with one tooth. 



Len(/th, xa-ths- Height, { th of an inch. 



Localiti/. Cor. Crag, Sutton. Recent, Scandinavia and Britain. 



This shell is by no means rare at the above locality. I have not yet seen it as a 

 fossil in any Formation of a more recent date. It appears to resemble in every respect 

 the living species, except perhaps it is a little larger. In order to institute a fair 

 comparison it may be thus more fully described. The posterior side is very short 

 and rounded, the anterior dorsal margin nearly straight, with a sharp and somewhat 

 prominent umbo. The shell is glossy externally, covered with radiating distant striae, 

 or rather small angular ridges, which extend all over the shell, but are most prominent 

 and conspicuous about the centre, and are more numerous on the older than on the 

 younger part of the shell, that is, they do not all terminate, or rather, originate at the 

 beaks ; an intermediate ray is occasionally introduced on the body of the shell, but 

 on the anterior slope ten or a dozen of these ridges are interposed between two which 

 proceed direct fropa the umbo : hinge with a tooth in the right valve, placed in the 

 direction of the dorsal margin anteriorly, having a depression on the upper side of it, 

 and a similarly formed tooth, with a corresponding cavity in the opposite valve ; 

 ligamental pit moderately large, sloping obliquely beneath the posterior dorsal margin. 

 In most of my specimens a slight depression is visible on the exterior, formed by the 

 contraction of the ventral margin, probably the result of a protruded byssus. 



It is considered in the recent state a deep-water animal, ranging from 10 to 140 

 Fathoms. 



4. MoNTACUTA FERRUGINOSA, Montague. Tab. XII, fig. 14, a, b. 



Mya FETiKXJGiNOSA. Mont. Test. Erit., Sup., pp. 22 & 16G, t. 26, fig. 2, 1808. 

 MoNTAcuTA FERRUGINOSA. Turt. Brit. Biv., p. CO, 1822. 



— — Alder. Cat. Moll. Nortli. and Durh., p. 96, 1848. 



— — Forb. and Hani. Hist. Brit. Moll., vol. ii, p. 72, pi. 18, 



figs. 5, 5a, bb, 1849. 



— — Alder. Ann. and Mag. Nat. Hist., 2d Series, vol. v, p. 210, 



pi. 6, B, 1850. 



— OBI.ONGA. Turt. Brit. Biv., p. 61, t. 11, figs. 11, 12, 1822. 



— — Flem. Brit. An., p. 465, 1828. 



— — Thorpe. Brit. Mar. Conch., p. 52, 1844. 



— — il/flcyiV/. Moll. Aberd., p. 302, 1843._ 



— GLABRA. Macffill. Moll. Aberd., p. 303, 1843. 



— — Thorjie. Brit. Mar. Conch., p. 245. 1844. 



17 



