90 EOCENE MOLLUSCA. 



is closely allied to A. barbatula ; it differs from the Barton A. appendiculata in being more 

 elongated and less inflated, and the siphon ilateral margin is more rounded, less angular, 

 and not so much produced. The rays also are more prominent in appendiculata, and the 

 lines of growth more distinct, giving to that shell a more elaborate ornamentation ; it has 

 also a larger or broader ligamental area ; the extremity of the hinge-line on the pedal 

 side is more angular, and there is a difference in the dentition. This species also appears 

 to attain to larger proportions. The only variation that I can detect between the English 

 shells and the French species, to which they are referred, is that the rays upon the siphonal 

 region of the Engish shells are not quite so broad as upon those from the Paris Basin. 

 The margin of the interior of our shell is slightly and irregularly denticulated. Consider- 

 able variation exists among specimens in regard to proportional dimensions, more particu- 

 larly in those from Huntingbridge ; in some the height is equal to three fifths of the 

 entire length, but in others the shell is very cylindrical, with a height not equalling half 

 the length. 



In the list of fossils from the Eocene deposits of this country, given by Mr. Prestwich 

 in his paper on the London Clay, published in the ' Journal of the Geol. Soc./ vol. iii, 

 p. 401, is the name of A. barbatula, as from Barton and Bracklesham. I have not seen a 

 specimen from any of our British deposits that can be safely referred to that species ; the 

 nearest approach to it are some of the elongated specimens of this species from Hunting- 

 bridge. 



15. Arca tegulata, Edwards, MS. Tab. XV, fig. 10, a, b. 



Spec. Char. A. testa elongatd, subcylindraced, depressd, lucidd, tenui, inequilateral i ; 

 obsolete costatd concentrice decussatd ; pedi-regione sub-attenuatd, siphoni-regione paulo 

 dilatatd, umbonibus minimis, remotis, prominulis ; area connexiis angustd, lanceolatd, 

 laevigata; dentibus — ? 



Shell elongate, somewhat cylindrical, depressed, glossy, thin, and inequilateral ; obso- 

 letely or lightly rayed, and decussated by lines of growth ; siphonal region a little the 

 broader; umbones small, remote and prominent; area for ligament narrow and smooth. 



Length, ,ths inch ; height, Jth inch. 



Localitg. Bracklesham {Edwards). 



This appears to be intermediate in form between A. angusta, Desh. (' Coq. foss. des 

 Env. de Par.,' 1. 1, p. 201, pi. 32, figs. 1 5, 16), and A. lucida, Desh. (' An. s. vert, du Bass. 

 dePar.,' t. 1, p. 891, pi. 67, figs. 26 — 28), it is nearer to the latter, but it is not so broad 

 in the siphonal region. Ours is an elegantly formed shell, and the only specimen I have 

 seen which has the two valves united is the one figured. The surface is ornamented with 

 rays irregularly distant ; those on the pedal region are narrow and close, increasing in 

 size as they approach the siphonal region, where they are broad and flat with a narrow 

 line between them ; these are crossed by a broad flat ridge of growth, which is smooth, 



