BIVALVIA. 89 
glabrd, valde inequilaterali ; pedi-regione brevi, rotundatd, siphoni-regione altiori, sub- 
cuneiformi vel obtuse angulatd ; umbonibus prominulis ; marginibus integris. 
Transversely oblong, convex, smooth, and glossy; pedal region the shorter; siphonal 
region obtusely wedge-shaped; front oblique; shell thin; beaks slightly prominent ; 
inner margin of valves smooth. 
Length, 3 inch ; height, 3ths of an inch, 
Localities. Hampstead (Wetherell), Primrose Mill (Adwards). 
“Some specimens have a few punctures, in which character they approach 4. 
impolita.’’—Sow. 
This species is not quite so abundant as 4. impolita, which it much resembles. The 
difference which appears to have caused the separation is a little more obliquity in the 
siphonal region of this species. The ventral margins appear to be free from crenulations, 
or at least, if they possessed them, they were very small and fine, and there is no sinuation 
for a byssus. The punctures spoken of by Mr. Sowerby arise from the want of continuity 
in the smoothness of the outer coating by which the radiating lines are seen distinctly 
separated, showing punctures or depressions between them and the decussating lines of 
growth. 
14, Arca pLanicosta, Deshayes. Tab. XIV, fig. 2, a, 6. 
ARCA PLANICOSTA. Desh. Coq. Foss. des Env. de Par., t. 1, p. 204, pl. 32, figs. 1, 2, 1824. 
— — D Orbigny. Prod. de Paléont., t. 2, p. 390, No. 1047, 1850. 
— — Desh. An. sans Vert. du Bass. de Par., t. i, p. 878, 1858. 
Byssoarca DUPLICATA, J. Sow., in Dixon’s Geol. of Suss., p. 93, pl. 3, fig. 22, 1850. 
Spee. Char. A. testé elongata, subcylindracea, convexiusculd, inequilaterali, tenui, 
in medio depressiusculd ; siphoni-regione valdé longiore, obtuse angulata ; pedi-regione 
converd, marginibus ventrali et dorsal subparallelis ; radiatim costellata costellis sepius 
planulatis, bifurcatis, aliquando granulatis ; umbonibus depressis ared ligaments angusta ; 
dentibus in medio minimis, utrdque extremitate obliquis. 
Shell. elongate, subcylindrical, inequilateral, siphonal region much the longer, central 
portion depressed ; pedilateral margin convex, siphonilateral margin obtusely angulated, 
dorsal and ventral margins nearly paraliel ; radiately costated with flattened and some- 
times divided rays; umbones depressed, ligamental area narrow ; teeth small in the 
centre, inclining towards each extremity. 
Length, 2 inches ; height, 1 inch. 
Localities. Bracklesham, Bramshaw, Brockenhurst, Brook (Zdwards). 
France, Senlis, Parnes, Valmondois, Auvers, Le Fayel (Deshayes). 
This is by no means rare at any of the localities cited, but I have not seen it from 
» Barton. It appears to correspond with the Paris Basin shell of the above name, and it 
