BIVALVIA. 85 
Length, sths inch; height, sths inch. 
Localities. Hampstead, Potter’s Bar, Highgate, Haverstock Hill (dwards and 
Wetherell). 
“Tt approaches 4. cucullaris, Desh. (vol. i, p. 206, pl. 38, figs. 1, 2, 3), but differs 
slightly in form as well as in the teeth being all transverse.” —J. Sowerby. 
All the specimens of this species that I have been able to see have the two valves 
united, and the dentition obscured ; it resembles 4. xitens of the same deposit, but is 
less oblique. 
A small individual in Mr. Wetherell’s cabinet (fig. 4, ¢) appears to be free from all 
exterior ornament ; it is possible that this may be the result of abrasion. There are also 
a few specimens of this genus in the same cabinet from the well at Hampstead, which are 
in a mutilated condition, and not sufficiently perfect for determination; they resemble the 
present species in shape, but appear to be more strongly and distinctly radiated. 
9. Arca interRupTa, Lamarck. Tab. XV, fig. 2, a, 6. 
Arca INTERRUPTA. Lamk. (non Poli). Ann. du Mus., t. 6, p. 220, No. 5, 1809. 
— —- Desh. Coq. Foss. des Env. de Par., t.i, p. 213, pl. 32, figs. 19, 20, 1824. 
— — Id. An. sans Vert. du Bass. de Par., t. i, p. 888, 1858. 
— —- Morris. Catal. Brit. Foss., p. 185, 1854. 
ByssoOarca INTERRUPTA. J. Sow., in Dixon’s Geol. of Suss., p. 93, t. 111, fig. 21, 1850. 
Spec. Char. A. testd obliqud, ovato-oblongd, compressa, valde inequilaterali ; costulatad 
et decussatd ; pedi-regione angustiore, siphoni-regione latiore et longiore ; cardine brevi, in 
medio edentulo, ad utramque extremitatem recurvo, pauci-dentato. 
Shell oblique, ovately oblong, compressed, very inequilateral ; striated and decussated by 
lines of growth; pedal region narrow and short; hinge area edentulous in the centre ; 
extremities with few oblique denticles ; ligamental area narrow ; umbones approximate. 
Length, 1 inch; height, } inch. 
Localities. Bracklesham, Selsey (Hdwards). 
France, Grignon, Parnes, Mouchy, Fontenay, Auvers (Deshayes). 
A fine series of this shell enriches the cabinet of Mr. Edwards. It is slightly com- 
pressed in the centre and contracted in the ventral margin opposite the umbo, but it has 
very little gape. ‘The hinge-area contains about six or seven denticles, those on the pedal 
side (three or four) are very slightly inclined to the hinge-margin, neither are the three or 
four on the siphonal side quite parallel with the dorsal edge. Between the striz or cost, 
which strongly denticulate the margins, there is an intermediate ray. Some of the French 
specimens are a trifle broader in the siphonal region than in the English shells, and the 
intermediate ray is scarcely so prominent. 
