A RCA. 



CONCHIFERA. 



jy) 



Coral Rag, Malton, Yorkshire. 



T. AuCA DUPHCATA. — The Twa-plaited Area, pi. LXXVII. 

 tig. 19. 



A. diipUeata. Sowerhy, V. pi. 47 ♦, fig. 1. 



Convex-ovate, transversely elongated, with double longitu- 

 dinal ril)s, which are furrowed along the middle ; margin 

 toothed ; marginal sinus obscure ; beaks a])j>n)ximatc. 



London Chiy, Ilordwell and Barton Cliff. 



8. AucA DEPEESSA. — The Depressed Area, pi. LXXVII. 

 fig. 24.. 



A. depressa. Sowerby, V. pi. 47+, fig. 2. 



Compressed, transversely elongated ; both sides rounded ; 

 surface with distant elevated crenulated strias, decussated by 

 lines of growth, the strize upon the anterior side very wide-set, 

 .■ippearing like knotted threads ; marginal sinus obscure. 



The Plastic Clay, Woolnich. 



9. Ahca tdmiba. — The Tumid Area, pi. LXXVII. fig. 15. 

 A. tumida. Sowerby, V. pi. 474, fig. 3. 



Very gibbose, the depth of each valve nearly equalling its 

 length, transversely elongated, with the anterior side acute ; 

 marginal sinus short and deep ; umbones remote, situate near 

 the posterior side ; surface with obsolete ribs. 



The Magnesian Limestone, Durham and Humbleton. 



10. Arca sub- acuta. — The Sub-acute Area, pi. LXXVII. 

 fig. 33. 



A. sub-aciUa. Sowerby, I. p. 95, pi. 44, upper figures. 



Gibbose, transversely oblong ; its breadth exceeding its 

 length ; hinge-line extending the whole kngtli of the valves; 

 surface longitudinally striated ; marginal plaits rounded, and 

 very deep, particularly at the anterior side ; teeth sharp and 

 numerous. 



The Chalk Marl, Hamsey. 



11. Arca carinata. — The Keeled Area, pi. LXXVII. 

 fig. 12. 



A. carinata. Sowerby, I. p. 96, pi. 45, lower figure. 



Very convex, parallelipe lal, its width twice its length ; 

 anterior side flattened, separated by an acute angle, truncated 

 at almost a right angle ; posterior side rounded ; surface lon- 

 gitudinally ribbed, every alternate one more prominent thau 

 the other. 



The Upper Greensand, Ilainpsiiire. 



12. Arca hotu-vdata. — The Rounded Arca, pi. LXXVII. 

 fig. 26. 



A. rotundata. Sowerby, Geo. Tr. 2il Ser. IV. p. 340, pi. 

 17, fig. 8. 



Transverse ; beaks small, quite contiguous ; anterior side 

 narrow ; posterior side large and ex]>anded, both extremities 

 rounded ; disk hollowed towards the base, where it is a little 

 concave ; surface with fine, radiating, lon:jitudinal stri.-e, and 

 a few concentric fine lines of growtli. 



The Greensand, Hlackdowu ami Lyme Regis. 



13. Arca impolita. — The Unpolished Arca. 



A. impolila. Sowerby, Geo. Tr. 2d Ser. V. p. 136, pi. s. 

 fig. 10. 



Transversely ovate ; very convex ; beaks small, hardly pro- 

 truding ; anterior side small, rounded ; posterior side larger 

 and rounded ; hingo ami basal lines parallel to each other ; 

 surface witli longitudinal lines of small puuctures ; substance 

 of the shell thin. 



The London Clay, Ilampstoad. 



14. Akca nitens. — Tlio Shining Arca. 



A. niteiis. Sowerby, Geo. Traus. 2d Ser. V. p. 136, pi. 8, 

 fig. 9. 



Transversely ovate, convex ; anterior side sm.all, somewhat 

 rounded; posterior a little wedge-shaped; surface smooth and 

 shining ; substauce of the siicU thin. 



In tlie London Clay, Ilampstead. 



15. Arca papillo.sa. — The Pimpled Arca, pi. LXXVII. 

 fig. 28. 



A. papulosa. Brown, Wernerian Mem. VIII. pi. 1, fig. 19. 



Transversely clougated ; beaks much produced, a little 

 curved inwards, and remote from each other ; hinge-line long 

 and straight, with numerous small teeth ; ligamental area 

 broad and elongated ; anterior side shortest and rounded ; 

 acute above ; posterior side obliquely truncated ; a ridge ema- 

 nating from the umbo terminates on the lower angle of the 

 truncations ; basal and hinge lines parallel ; surface with nume- 

 rous divergent, longitudiual, papillose ribs ; and a few strong 

 transverse lines of growth. 



The Pleistocene Marine Formation, Portrush, Ireland. 



16. Aeca costata.— The Ribbed Arca, pi. LXXVII. f. 32. 

 A. costata. Brown, Mem. Man. Geo. Soc. I. p. G6, pi. 6, 



figs. 34, 35. 



Transversely oblong-ovate ; anterior side very short and 

 acute ; tlie posterior lengthened and sub-truncated ; beaks 

 small and slightly produced ; surface with three or four pro- 

 minent ribs emanating from the beaks and termiuating on the 

 posterior side, with many uearlyobsolete concentric^wriukles. 



The New Red Sandstone, Newton, Manchester. 



17. Arca .emula. — Emulating Area, pi. LXXVII. fig. 36. 

 A. winiila. Phillips, Geo. York, I. pi. 3, fig. 29. 

 Transversely elongated, oblique; bjaks large, obtuse, and 



approximating; disk slightly hollowed; base a little concave 

 in the centre ; surface with divergent, longitudinal stria;, in- 

 terrupted by the transverse lines of growth, which are pretty 

 strong, and following the concave direction of the basal line. 

 The Coral Rag, Malton, Yorkshire. 



18. Arca lactea. — The Jl ilk- white Arca. 



A. lactea. Brown, lUust. Conch. Gt. Brit, and Ird. p. 86, 

 p'. XXXIIL fig. 6. 



Transversely oblong, and slightly oblique ; beaks obtuse, 

 remote; posterior side a little anguJated ; somewhat open at 

 the base for the passage of a byssus ; longitudinally ribbed, 

 and crossed by numerous lines of growth, producing a decus- 

 sated aspect ; with the interstices punctured ; margin plain. 



The Pleistocene Marine Formation, in the Fortli. 



19. AucA ELONGATA. — The Elongated Area, pi. LXXVII. 

 fig. 34. 



Transversely elongated ; oblique ; beaks very obtuse ; liga- 

 mental area of medium width ; anterior side angular above 

 and rounded below ; posterior sub-truncated obliquely, rather 

 acute above, and rounded below ; hinge-line and base nearly 

 parallel, the latter a little concave in the centre ; surface with 

 fine divergent, longitudinal strias ; crossed by remote, rather 

 regular lines of growth. 



In the Greensand, Blackdown. 



20. Arca cylindrica. — The Cylindrical Arca. 



A. cylindrica. Portlock, Geo. Rep. p. 4?8, pi. 34, fig. 9. 



