80 EOCENE MOLLUSCA. 



2. Arc a aviculina? Deshayes. Tab. XV, tig. 7, a, b. 



Arca aviculina. Desk. An. sans Vert, du Bass, de Par., t. i, p. 88", pi. 66, figs. 15 — 17, 

 1858. 



Spec. Char. A. testa elongatd, angusto-subcylindraced, depressiusculd, incequilaterali, 

 radiatim eleganterque costettatd ; costellis inaqualibus , in medio tenuibus ; extremitatibus 

 crescentibus, elevalis, subimbricatis ; umbonibws depressis, brevibus ; area connexus parvd ; 

 area dentali paucidentatd. 



Shell slender, elongate, subcylindrical, somewhat depressed, inequilateral, elegantly 

 covered with rays or small ribs ; rays smaller or thinner in the centre, enlarging towards 

 the lateral margins ; beaks small, depressed ; area of the connector narrow ; dental margin 

 sparingly furnished. 



Length, 1§ inch ; height, #hs. 



Localities. Bracklesham, Bramshaw, Brook {Edwards), Huntingbridge {Fisher). 



This is an elegant species, and, I believe, not very rare. There is considerable 

 difference between our shell and the figure of the French species, to which it is here 

 doubtfully referred ; but I feel unwilling to separate them upon what do not appear to be 

 essential distinctions. There is also a difference between the British specimens from 

 different localities, those from Bramshaw and Brook being more delicately rayed than 

 those from Huntingbridge and Bracklesham. In general, our shell appears to be less 

 cylindrical than that of the Paris Basin species ; but in this character our own specimens 

 vary materially. The principal difference is in the position of the umbo, which is more 

 eccentric in the British than in the French shell, and in this character the former more 

 closely approaches A. interrupta, where the siphonal region is also broader or higher ; but 

 it differs from that species in being longer, differently rayed, and in having the central 

 portion somewhat compressed, with an inflated or tumid siphonal region. l 



3. Arca biangula, Lamarck. Tab. XIV, Fig. 1, a—f. 



Arca biangula. Lam. An. du Mus., t. vi, p. 219, 1809, and t .ix, pi. 19, fig. 4, a, b, 1824. 



— — Desk. Coq. Foss. des Env. de Par., t. i, p. 198, pi. 34, figs. 1—6, 1824. 



— Branderi. /. Sowerby. Min. Conch., t. 276, figs. 1, 2, 1821. 



— hyantula. Desk. Coq. Foss. des Env. de Par., t. i, p. 199, pi. 34, figs. 7, 8, 1824. 



— — Gold/. Petr. Germ., vol. i, p. 143, t. 122, fig. 3, a-d, 1826. 

 Byssoarca Branderi. J. Sow., in Dixon's Geol. Sussex, pp.92, 169, t. Ill, fig. 23, 1850. 



1 Since the above has been in press, I have seen the description and figure of Arca Laekeniana, Le 

 Hon. (' Descr. succ. de quelq. nouv. esp. des terr. tert. Eoc. des env. de Brux.,' p. 7, No. 15), a species which 

 more resembles our fossil than does the one to which it is here doubtfully assigned, but the rays upon our 

 shell are somewhat different ; they are apparently more distant, and have intermediate striae, which are 

 neither shown nor said to exist upon the Belgian fossil. I think, however, that when specimens of each 

 can be compared, they may be found to be identical. 



