224 
CONCHIFERA. 
Ampiiidesma- 
Ovate, somewhat depressed, nearly equilateral; anterior 
side rather elongated, gaping very slightly, and very little 
truncated ; front rounded ; beaks much depressed and obtuse; 
surface smooth, and somewhat polished internally. 
The London Clay, Plumstead, near Woolwich, Kent. 
8. Potomomya sub-angulata.— The Sub-augulated Poto- 
momya, pi. XC1I. fig. 30. 
Mya sub-angulata. Sowerby, I. p. 174, pi. 76, fig. 3. 
Transversely oblong ovate, somewhat compressed, nearly 
equilateral ; anterior side angulated above, and a little acu¬ 
minated and slightly gaping ; front a little marginate ; cardinal 
tooth very large; surface smooth. 
The London Clay, Barton. 
Family IV.—MACTRACEA. 
Shells equivalve, often gaping at the lateral extremities; 
ligament interior, or partly external ; animal with the foot 
small and compressed. 
Sub-Division I.—Ligament seen externally, or double. 
Genus LIV.—AMPHIDESMA.— Lamarck. 
Shell equivalve, transverse, slightly inequilateral, somewhat 
ovate or orbicular; some species gaping at the sides; each 
valve provided with one, or, in some instances, two small, 
slender, cardinal teeth, and two distinct, elongated, lateral 
teeth, situate near the hinge in one valve, and are nearly ob¬ 
solete in the other ; paliial impression with a very large sinus; 
ligament double, its external portion slender, and rather short, 
and the internal cartilage generally longer and larger, adhe¬ 
rent in both valves to an elongated groove or pit, which varies 
in length in different species, and takes its rise immediately 
within the umbo, and is prolonged within the anterior lateral 
tooth. 
1. Ampiiidesma tenuistriatum. —The Thin-striated Am- 
phidesma, pi. XCI. fig. 48. 
A. tenuistriatum. Sowerby, Geo. Tr. 2d Ser. IV. p. 314, 
pi. 16, fig 7. 
Transversely elongated, sub-ovate, very flat, posteriorly 
sub-truncated; beaks nearly central, very obtuse; surface 
with numerous concentric striae. 
The Greensand, Blackdown. 
2. Ampiiidesma skcuriforme.— The Hatchet-shaped Am- 
phidesma, pi. XCI. fig- 3?. 
A. securiforme. Phillips, Geo. York, I. pi. 7, fig- 10. 
Transversely oblong ovate; beaks obtuse, nearly central, 
narrowed and rounded at both extremities ; surface smooth, 
with remote lines of growth. 
The Inferior Oolite, Glaizedale. 
3. Ampiiidesma deltoide. —The Deltoidal Ampiiidesma, 
pi. XCI. fig. 39. 
A. deltoide . Portlock, Geo. Sur. p. 439, pi. 36, fig. 7- 
Nearly orbicular, much inflated, regularly rounded ante¬ 
riorly ; sharply truncated posteriorly ; a well defined diagonal 
ridge proceeds from the beak to the margin. 
The Carboniferous Limestone, Derry and Tyrone. 
4. Ampiiidesma axiniformis. —The Axe-shaped Amphi- 
desma, pi. XCI. fig. 41. 
A. axiniformis. Portlock, Geo. Rep. p. 439, pi. 36, fig. 6. 
Nearly triaugular; beaks almost central ; truncated poste¬ 
riorly, and straight anteriorly; surface smooth. 
The Carboniferous Limestone, Clogher, Tyrone, Ireland. 
5. Ampiiidesma album. —The White Ampiiidesma, pi. 
XCI. fig. 46. 
A. album. Fleming, Brit. An. p. 432. Mactra alba. 
Wood, Linn. Trans. VI. p. 174, pi. 16, figs. 9, 10. 
Transversely ovate, sub-triangular, rounded at both extre¬ 
mities ; beaks sub-central; surface smooth. 
The Mammiferous Crag, Bulcham; the Red Crag, Bawd- 
say, and Coral Crag, Sutton. 
6. Ampiiidesma carbonarium. —The Coal Ampiiidesma, 
pi. XCI. figs. 44, 45. 
Venus carbonarium . Sowerby, Geo. Tr. 2d Ser. IV. 
pi. 39, fig. 2. 
Nearly orbicular, very convex, most so towards the beaks ; 
anterior side rounded ; posterior side truncated, and rather 
square ; beaks rather prominent, inflected, and remote. 
The Coal Measures, Coalbrookdale. 
7. Ampiiidesma Portlockii. —Portlock's Ampiiidesma, 
pi. XCI. fig. 40. 
A. carbonaria. Portlock, Geo. Rep. p. 438, pi. 36, fig. 8. 
Sub-orbicular, rounded anteriorly ; very slightly truncated 
posteriorly, with a slight inflection of the margin below the 
truncation ; beaks nearly central and obtuse ; surface appears 
to have been concentrically and finely striated. 
The Carboniferous Limestone, Clogher, Tyrone. 
8. Ampiiidesma prismaticum. —Prismatic Ampiiidesma, 
pi. XCI. fig. 47. 
A. prismatica. Brown, Ill. Rec. Conch. Brit. p. 105, pi. 
42. fig. 5. 
Transversely oblong ovate, much compressed, thin, and fra¬ 
gile ; rounded anteriorly, and acuminated posteriorly, with a 
slight oblique sub-truncation ; surface with very minute con¬ 
centric stria?. 
The Coral Crag, Sutton. 
9. Ampiiidesma recurvum. —The Recurved Ampiiidesma, 
pi. XCI. fig. 49. 
A. recurvum. Phillips, Geo. York, pi. 5, fig. 25. 
Transversely oblong oval ; beaks large, produced, and sub- 
central, both sides somewhat recurved; surface smooth, with 
shallow lines of growth. 
The Coral Rag, Malton, and the Kelloways Rock, near 
Scarborough. 
Sub-Division II.—Shell not gaping at the side; ligament 
external. 
