PoTOMOMYA. 



CONCHIFERA. 



223 



17. CoRBCLA DEPHESSA. — Depressed Corbula,pl. XCI. f. 5. 

 C. depressa. Phillips, Geo. York, I. pi. 9, fig. 16. 

 Sub-orbicular ; beaks nearly central and much produced ; 



slightly arcuated posteriorly from the beaks downwards ; an- 

 teriorly finely rounded ; the basal lino arcuated ; surface with 

 regular concentric ridges, and a few lines of growth. 

 The Great Oolite, Cloughton Wyke, Yorkshire. 



18. CoRBULA Hennahii. — Henuah's Corbula, pl.XCI. f. 10. 

 Sowerby, Goo. Tr. 2d Ser. V. pi. jO, fig. 1. 

 Transversely elongated, ovate ; posterior side projecting 



into a short beak ; moderately convex ; valves rather unequal ; 

 posterior side obliquely truncated ; surface smooth. 

 The Devonian Rocks, Plymouth. 



19. CoRBOLA Ficus.— Tho Fig Corliula, pi. XCI. fig. 12. 

 Solenjieus. Brander, fig. 103. 



Orbicular, with tho posterior side projecting into a beak ; 

 whole surface with strong transverse ribs ; beaks obtuse. 

 The London Clay, Barton. 



20. Corbula alata. — The Winged Corbula, pi. XCI. 

 fig. 3-i. 



C. alata. Sowerby, Geo. Tr. 2d Ser. IV. p. 176, pi. 21, 

 fig. 5. 



Sub-orbicular, convex ; anteriorly rounded ; posteriorly 

 contracted and truncated ; surface smooth ; beaks incurved. 



The Weald, Pouncefield, Burwash. 



21. Corbula costata. — Tho Ribbed Corbula, pi. XCI. 

 figs. 26, 27, 28. 



C. revoluta. Var. B. Sowerby, III. p. 16, pi. 209, figs. 

 11, 12, 13. 



Transversely oblong ; tumid ; anterior side narrowed, pro- 

 duced, pointed, and obliquely truncated ; surface with a few 

 deep transverse furrows. 



The London Clay, Barton Cliff. 



22. Corbula truxcata. — The Truncated Corbula, pi. 

 XCI. figs. 32, 33. 



C. truncata. Sowerby, Geo. Tr. 2d Ser. IV. p. 341, pi. 16, 

 fig. 8. 



Transversely oblong ovate ; beaks large, nearly central ; 

 posterior side produced, obliquely truncated, and pointed to- 

 wards the front ; surface transversely striated. 



The Greensand, Blackdown. 



23. Corbula punctum. — The Punctured Corbula, pi. XCI. 

 fig. 36. 



C. punctum. Phillips, Geo. York, I. pi. 2, fig. 6. 



Triangular, convex, oblique ; beaks produced ; sides nearly 

 straight ; surface with fine, longitudinal, and transverse stria;» 

 which, without the aid of a lens, seem to bo punctures. 



The Speeton Clay, Spectcn, Yorkshire. 



24. Corbula limosa. — The Mudd Corbula. 

 C. limosa. Fleming, Brit. An. p. 426. 



Transversely sub-triangular, and longitudinally heart- 

 shaped; beaks gibbous; surface slightly grooved by the lines 

 of growth ; shell thin. 



The Carboniferous Limestone, Scotland. 



25. Corbula cardioides. — The Cardium-like Corbula, pi. 

 XC. fig. 42. 



C. cardioides. Phillips, Geo. York, I. pi. 14, fig. 12. 

 Mactromga glol/ofa. Agassiz et Crit. (Myes) pi. 9 d, figs. 

 9-14, (?) 



Slightly transversely ovate, much inflated ; anterior side the 

 larger, and rounded ; posteriorly shorter and truncated ; beaks 

 large, and greatly produced and incurved ; surface smooth, 

 with regular, almost oqui<listant lines of growth. 



The Lias, Robin Hood's Bay and Cheltenham. 



Genus LII.— N^ARA— Gmy. 



Transversely oblong ovate ; shell very convex ; posterior 

 side large and rounded ; anterior side abruptly tapering to a 

 lengthened and acuminated beak-like elongation ; beaks small, 

 inflected ; hinge-teeth with one large, elevated, and recurved 

 cardinal tooth in the right valve, which fits into a pit under 

 tho edge of the superior margin of the left valve; cartilage 

 attached in central pits beneath the beaks ; two muscular im- 

 pressions in each valve ; pallial impression obsolete. 



1. N.eara dispar. — The Difl'ering N;eara, pi. XCIII. f. 21 . 



Corbula dis par. Deshayes, Coq. Foss. p. 57, pi. 18, figs. 

 36, 37, 38. 



Transversely and acutely oval, thin, biangulated in front, 

 and acuminated into a beak-like elongation ; tho right valve 

 concentrically furrowed, and the left smooth. 



In the London Clay, Barton. 



Genus LIII.— POTOMOMYA.— /. Sowerby. 



Shell sub-triangular, inequivalve, gaping, and generally sub- 

 truncated at the anterior side ; left valve encompassing the 

 other all round, receiving its edges upon the thickened parts 

 on each side of the hinge; right valve with a large, erect, 

 spoon-shaped double tooth ; left valve with small hollow for 

 the reception of tho ligament ; pallial impression with a small 

 rounded sinus, forming a quarter of a circle, situate close to 

 the anterior muscular impression. 



The remote tooth, with its accompanying hollow, forming a 

 secure nest from the edges of the opposite valves ; the inequal- 

 lity of the valves, and tho form and situation of tho sinus, 

 are the chief characters which distinguish this genus from 

 that of Mya. 



1. PoTOMOMYA GREGARiA. — The Grcgarcous Potomomya, 

 pi. XC. figs. 8 and 10. 



P. gregaria. Sowerby, IV. p. 87, pi. 383. 



Sub-triangular, its breadth being about one and a half its 

 length ; anterior side of the right valve slightly produced and 

 truncated ; left valve somewhat larger than the other, .and re- 

 ceiving it within its entire margin upon the thickened parts 

 on each side of the hinge, with the lower margin a little in- 

 curved ; posterior side with a remote tooth, and with a slight 

 furrow within the anterior edge; beaks depressed and obtuse ; 

 surface smooth. 



Fresh Water, top of Headou Hill, Isle of Wight, and Cai- 

 bourne. 



2. Potomomya plana. — The Plain Potomomya, pi. XC'IF. 



fig. 31. 



J/j/rt plana. Sowerby, I. p. 173, pi. 76, fig. 2. 



