22i 



CONCHIFEUA. 



A> 



MPHIDESM*. 



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-aDgulated Poto- 

 momya, pi. XCII. fig. 30. 



Mya snh-angulata. Sowerby, I. p. 174, pi- 7C, fig. 3. 



Transversely oblong ovate, somewhat compressed, nearly 

 equilateral ; anterior side angulated above, and a little acu- 

 minated and slightly gaping ; front a little marginatc ; 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 L — Ligament seen externally, or double. 



Genus LIV.— AMPHIDESMA.— iff7«a/(r>t. 



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 ; pallial 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 diflerent species, and takes its rise immediately 

 within the umbo, and is prolonged within the anterior lateral 

 tooth. 



1. Amphidesma tendistriatitm. — The Thin-striated Am- 

 phidesma, pi. XCI. fig. 48. 



A. tenaistriatum. 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 stricC. 



The Greensand, Blackdown. 



2. Amphidesma securiforme. — The Hatchet-shaped Am- 

 phidesma, pi. XCI. fig. 37. 



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. Amphidesma deltoide — The Deltoidal Amphidesma, 

 pi. XCI. fig. 39. 



A. deltoide. Portlock, Geo. Sur. p. 489, pi. 36, fig. 7. 



Nearly orbicular, much inflated, regularly rounded ante- 

 riorly ; sharply truncated posteriorly ; a well dc fined diagonal 

 ridge proceeds from the beak to the margin. 



The Carboniferous Limestone, Derry and Tyrone. 



4. Amphidesma axinipormis. — The Axe-shaped Amphi- 

 desma, pi. XCI. fig. 41. 



A. axiniformis. Portlock, Geo. Rep. p. 430, pi. 36, fig. 6. 

 Nearly triangular ; beaks almost central ; truncated poste- 

 riorly, and straight anteriorly ; surface smooth. 



The Carboniferous Limestone, Clogher, Tyrone, Ireland. 



5. Amphidesma album. — The White Amphidesma, pi. 

 XCI. fig. 46. 



A. album. Fleming, Brit. An. p. 432. Mactra alia. 

 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. Amphidesma carbonakium. — The Coal Amphidesma, 

 pi. XCI. figs. 44, 4-,. 



Venus carhonarium. 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. Amphidesma Portlockii. — Portlock's Amphidesma, 

 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. Amphidesma prismaticcm. — Prismatic Amphidesma, 

 pi. XCI. fig. 47. 



A. prismatica. Brown, 111. 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 striae. 



The Coral Crag, Sutton. 



9. Amphidesma recuevum. — The Recurved Amphidesma, 

 pi. XCI. fig. 49. 



A. recurviim. 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. 



