BIVALVIA. 85 



Length, f ths inch ; height, |ths inch. 



Localities. Hampstead, Potter's Bar, Highgate, Haverstock Hill {Edwards and 

 Wetherell). 



"It approaches A. cucullaris, Desh. (vol. i, p. 206, pi. 33, 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 A. nitens of the same deposit, but is 

 less oblique. 



A small individual in Mr. Wetherell's cabinet (fig. 4, c) 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, b. 



Arca interrupta. Lamk. (non Poli). Ann. du Mu8., t. 6, p. 220, No. 5, 1809. 



— — Desh. Coq. Foss.desEnv. dePar., t.i,p. 213, pi. 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. 



Byssoarca interrupta. /. Sow., in Dixon's Geol. of Suss., p. 93, t. Ill, fig. 21, 1850. 



Spec. Char. A. testa obliqud, ovato-oblongd, compressd, valde inaquilaterali ; costulatd 

 et decussatd ; pedi-regione angustiore, siphoni-regione latiore et longiore ; cardine brevi, in 

 medio edenf/do, ad utramque extremitatem recurvo, panci-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 {Edwards). 



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 striae or costae, 

 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. 



