PoTOMOMYA. 
OONCHIFEltA. 
17. Corbula depressa. —Depressed Corbula,pi. XC1. f. 5. 
C. depressa. Phillips, Geo. York, I. pi. 0, lig. 16. 
Sub-orbicular; beaks nearly central and much produced; 
slightly arcuated posteriorly from the beaks downwards ; an¬ 
teriorly finely rounded ; the basal line arcuated ; surface with 
regular concentric ridges, and a few lines of growth. 
The Great Oolite, Cloughton Wyke, Yorkshire. 
18. Corbula Hennahii. —Ilennah’s Corbula, pl.XCI. f. 10. 
Sowerby, Geo. Tr. 2d Ser. V. pi. .56, 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. Corbula ficus. —The Fig Corbula, pi. XCI. fig. 12. 
Solen ficus. Brander, fig. 103. 
Orbicular, with the 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. 34. 
C. alata. Sowcrby, 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, Pounccfield, Burwash. 
21. Corbula costata. —The 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 truncata.—T he 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 punctual —The Punctured Corbula, pl.XCI. 
fig. 36. 
C. punclum. Phillips, Geo. York, I. pi. 2, fig. 6. 
Triangular, convex, oblique ; beaks produced ; sides nearly 
straight; surface with fine, longitudinal, and transverse striae 
which, without the aid of a lens, seem to bo punctures. 
The Speeton Clay, Specten, 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-liko Corbula, pi. 
XC. fig. 42. 
C. cardioides. Phillips, Geo. York, I. pi. 14, fig. 12. 
Maetromga globosa. Agassiz et Crit. (Myes) pi. 9 6?, figs. 
9-14, (?) 
223 
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 equidistant lines of growth. 
The Lias, Robin Hood’s Bay and Cheltenham. 
Genus LII.—N^EARA —Gray. 
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 
the 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 Differing Naeara, pi. XCIII. f. 21. 
Corbula dispar. Deshayes, Coq. Foss. p. 57, pi. 18, figs. 
36, 37, 38. 
Transversely and acutely oval, thin, biangulated in front, 
and acuminated into a beak-liko elongation; the right valve 
concentrically furrowed, and the left smooth. 
In the London Clay, Barton. 
Genus LIU.—POTOMOMYA.— J. 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 the 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¬ 
ity of the valves, and the form and situation of the sinus, 
aro the chief characters which distinguish this genus from 
that of Mya. 
J. PoTOMOMYA gregaria. —The Gregareous 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 sido 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 
ou 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 lleadon Hill, Isle of W iglit, and Cal- 
bourne. 
2. Potomomya plana.— The Plain Potomomya, pi. XCH. 
fig. 31. 
Mya plana. Sowerby, I. p. 173, pi. 76, fig. 2. 
