BIVALVIA. 129 
Spec. Char. Testa minutd, transversd, obliqud, ovatd, valdé inequilaterali, convead, 
politd, tenui, fragili ; anticé productd, 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. 
Length, 2;ths. Height, 1th of an inch. 
Locality. Cor. Crag, Sutton. Recent, Scandinavia and Britain. 
This shell is by no means rare at the above locality. I have not yet seen it asa 
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 from 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 rerrucinosa. Mont. Test. Brit., Sup., pp. 22 & 166, t. 26, fig. 2, 1808. 
MonTacuTa FERRUGINOSA. Turt. Brit. Biv., p. 60, 1822. 
—_ _ Alder. Cat. Moll. North. and Durh., p. 96, 1848. 
—~ — Ford. and Hani. Hist. Brit. Moll., vol. ii, p. 72, pl. 18, 
figs. 5, 5a, 56, 1849. 
a — dider. Ann, and Mag. Nat. Hist., 2d Series, vol. v, p. 210, 
pl. 6, B, 1850. 
— optonGa. Turt. Brit. Biy., p. 61, t. 11, figs. 11, 12, 1822. 
— — Flem. Brit. An., p. 465, 1828. 
— a Thorpe. Brit. Mar. Conch., p. 52, 1844. 
— _ Maegill. Moll. Aberd., p. 302, 1843. 
— GLABRA. Macgill. Moll. Aberd., p. 303, 1843. 
— _— Thorpe. Brit. Mar. Conch., p. 245, 1844. 
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