226 



CONCHIFERA. 



Mactrina. 



proximate, and incurved ; margin plain ; posterior edge 

 rounded. 



In the Lower Greensand, at Parham Park, Sussex, and at 

 Shanklin Cbine, Isle of Wight ; in the Lower Greensand, 

 Lyme Regis and Biackdown. 



2. Thetis major. — The Large Thetis, pi. XCII. figs. 1, 2. 



T. major. Sowerby, VI. p. 20, pi. 513, figs. 1, 2, 3, 4. 



Orbicular, or oblong oval, very convex ; beaks produced, 

 and nearly central, much incurved and approximate ; poste- 

 rior side rather angular ; surface smooth. 



The Upper Greensand, Devizes and Biackdown ; and the 

 Lower Greensand, Isle of Wight, North Wiltshire, &c. 



It will be seen from our figures that there is considerable difference 

 of form in the species. 



Sub-Division III.- 



sides. 



-Ligament internal ; shell gaping at the 



Genus LIX. — MACTRA. — Linnaeus. 



Shell generally thin, sometimes thick ; equivalve — for the 

 most part nearly equilateral, and more or less regularly trian- 

 gular, slightly gaping at one end, and almost imperceptibly so 

 at the other; each valve with one V-shaped cardinal tooth, 

 the point being next the umbo, and diverging from it, and in 

 some species the limbs are disunited at the base, so as to give 

 the appearance of two distinct teeth ; close on the posterior 

 side is situate a very thin sharp tooth ; immediately behind 

 the angular tooth is situate the pit for the reception of the liga- 

 ment, and projecting somewhat within the shell ; one valve 

 with two lateral teeth on each side, and one on both sides in 

 the other, diverging from the beaks, placed near the margin of 

 the shell, and fitting into the space between the two in the 

 opposite valve ; two lateral, remote, muscular impressions ; 

 mantle muscular iaipressinn with a small sinus ; ligament con- 

 sistiug of two portions, the one considerably larger than the 

 other, and internal, and the other half external. 



1. Mactra angulata. — The Augulated Mactra, pi. XCI. 

 fig. 37*. 



M. angulata. Sowerby, Geo. Tr. 2d Ser. IV. p. 341, pi. 

 16, fig. 9. 



Nearly triangular, convex ; posterior side defined by a 

 ridge ; beaks small, nearly central, and approximating ; sur- 

 face smooth. 



The Greensand, Biackdown. 



2. Mactra striata. — The Striated Mactra, pi. XCI. f. 42. 

 M. striata. Brown, Wern. Mem. VIII. p. 93, pi. I.f. 22. 

 Sub-triangular, convex, with nearly equal sides ; beaks 



central, and slightly turned to one side ; lateral teeth promi- 

 nent ; surface with very strong concentric striie. 



The Pleistocene Marine Formation, Stevenston, Ayrshire. 



3. Mactra depressa. — Depressed Mactra, pi. XCI. f. 51. 



M. depressa. Deshayes, Coq. Foss. p. 31, pi. 4, fs. 11-14. 



Shell thin, trigonal, depressed ; umbones somewhat promi- 

 nent ; cardinal teeth simple, and not plicated ; lateral teeth 

 close to the cardinal ones ; lunule depressed and plain. 



The London Clay, Bracklesham. 



4. Mactra arcuata. — The Arcuated Mactra, pi. XCI. f. 56. 

 M. arcuata. Sowerby, p. 135, pi. 160, figs. 1 and 6. 

 Ovate, length equal to about four-fifths its width ; both 



sides arched, the posterior one smallest ; hinge narrow ; late- 

 ral teeth striated ; surface smooth, with a few well-defined 

 lines of growth. 



In the Mammiferous, Red, and Coralline Crags, Sutton. 



5. Mactra solida. — The Strong Macra, pi. XCI. figs. 

 53, 57, 58. 



M. ovalis. Sowerby, II. p. 136, pi. 160, fig. 5. Brown's 

 Illust. Rec. Conch. Brit. p. 108, pi. 41, figs. 3, 4. 



Sub-triangular, strong ; sides nearly equal ; surface smooth, 

 with a few concentric wrinkles. 



The Mammiferous Crag, Thorpe ; the Red and Coral Crags, 

 Sutton. 



6. Mactra sub-truncata. — The Sub-truncated Mactra, pi. 

 XCI. fig. 43. 



M. cuneata. Sowerby, II. p. 136, pi. 1 60, fig. 7. Mactra 

 sub-truncata. Brown, Illust. Rec. Conch. Great Britain and 

 Ireland, p. 108, pi. XCI. fig. 43. 



Sub-triangular, inequilateral, strong, and moderately con- 

 vex ; anterior side rounded ; posteriorly somewhat acumin- 

 ated and flattened ; surface with strong transverse striae. 



The Mammiferous Crag, Thorpe, and Red Crag, Sutton. 



7. Mactra deaurata. — The Gilded Mactra, pi. XCI. f. 53. 

 M. deaurata. Turton, Brit. Biv. p. 71, pi. 5, fig. 8. 

 Oblong oval, inequilateral, rather fiat ; beaks obtuse and 



incurved, placed a little to the posterior side, which is sub-trun- 

 cated ; anteriorly rounded ; surface smooth. 

 The Red Crag, Sutton. 



8. Mactra glauca. — The Grey Mactra, pi. XCI. fig. 50. 



M. fflauca. Brown, Illus. Rec. Conch. Brit. pi. 41, fig. 1. 



Sub-triangular, convex, thin ; beaks central, obtuse, and in- 

 flected ; anterior side slightly wrinkled ; surface with very 

 fine concentric strise. 



The Red Crag, Sutton. 



9. Mactra stultorum. — The Foolish Mactra, pi. XCI. f. 55. 

 M. stultorum. Brown, Illus. Rec. Conch. Brit. p. 108, pi. 



41, fig. 2. 



Sub-triangular, thin, moderately convex ; beaks central, 

 rather prominent, and inflected ; sides nearly equal ; surface 

 with very fine, rather irregular, concentric striae. 



The Mammiferous Crag, Thorpe, and the Red Crag, Sutton. 



9. Mactra truncata. — The Truncated Mactra, pi. XCI. 

 fig. 55. 



J\L truncata. Brown, Illust. Rec. Conch. Brit. p. 1 08, pi. 

 41, fig. 5. 



Triangular, moderately convex, strong, and thick ; sides 

 nearly equal and straight ; surface smooth, with a few obsolete 

 lines of growth. 



Pleistocene Marine Formation, Ayrshire, and Frith of 

 Forth. 



Genus LX.— MACTRINA.— ^rown. 



Shell sub -triangular, equivalve, nearly equilateral; beaks 



