BIVALVIA. 89 



glabra, valde incequilaterali ; pedi-regione brevi, rotundatd, siphoni-regione altiori, sub- 

 cuneiformi vel obtuse angulat a ; 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, ^ inch ; height, T 3 5 ths of an inch. 



Localities. Hampstead (Wetherell), Primrose Hill {Edwards). 



" Some specimens have a few punctures, in which character they approach A. 

 impolita." — Sow. 



This species is not quite so abundant as A. 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 abyssus. 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, Beshayes. Tab. XIV, fig. 2, a, b. 



Arca planicosta. Desk. Coq. Foss. des Env. de Par., t. i, p. 204, pi. 32, figs. 1, 2, 1824. 

 WOrbigmj. Prod, de Paleont., t. 2, p. 390, No. 1047, 1850. 

 — — Desk. An. sans Vert, du Bass, de Par., t. i, p. 878, 1858. 



Byssoarca duplicata. /. Sow., in Dixon's Geol. of Suss., p. 93, pi. 3, fig. 22, 1850. 



Spec. Char. A. testa elongatd, subcylindraced, convexiusculd, inaquilaterali, tenui, 

 in medio depressiusculd ; siphoni-regione valde longiore, obtuse angulatd ; pedi-regione 

 convexd, marginibus ventrali et dorsali subparallelis ; radiatim costellatd costellis scepius 

 planulatis, bifurcatis, aliquando granulatis ; umbonibus depressis area ligamenti angustd ; 

 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 parallel ; 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 (Edwards). 



France, Senlis, Parnes, Valmondois, Auvers, Le Fayel (Beshayes). 



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 



