226 
CONCHIFERA. 
Mactrlna. 
proximate, and incurved; margin plain; posterior edge 
rounded. 
In the Lower Greensand, at Parham Park, Sussex, and at 
Shanklin Chine, Isle of Wight; in the Lower Greensand, 
Lyme Regis and Blackdown. 
2. Tiietis major. —The Large Thetis, pi. XCII. figs. 1 , 2. 
T. major. Soworby, VI. p. 20, pi. 513, figs. 1 , 2, 3, 4. 
Orbicular, or oblong oval, very couvex; beaks produced, 
and nearly central, much incurved and approximate; poste¬ 
rior side rather angular ; surface smooth. 
The Upper Greensand, Devizes and Blackdown; 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.—Ligament internal; shell gaping at the 
sides. 
Genus LIX.—MACTRA. — Linnaeus. 
Shell generally thin, sometimes thick ; equivalve—for the 
most part uearly equilateral, and more or less regularly trian¬ 
gular, slightly gaping at one end, and almost imperceptibly so 
at the other; each valve with one Y-shaped cardinal tooth, 
the point being noxt 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 situato 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 impression with a small sinus ; ligament con¬ 
sisting 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. 
1G, fig. 9. 
Nearly triangular, convex ; posterior side defined by a 
ridge ; beaks small, nearly central, and approximating; sur¬ 
face smooth. 
The Greensand, Blackdown. 
2. M vctra 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 striae. 
The Pleistocene Marine Formation, Stovenston, Avrshire. 
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 aud 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. 13G, pi. 1G0, 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. 
G. Mactra sub-truncata. —The Sub-truncated Mactra, pi. 
XCI. fig. 43. 
M. cuneata. Sowerby, II. p. 13G, pi. 160, fig. 7. Mactra 
sub-truncata. Brown, Illust. Roc. 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 flat; 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. glauca. Brown, Ulus. 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 stria?. 
The Red Crag, Sutton. 
9. Mactra stultorum. —The Foolish Mactra, pi. XCI. f. 55. 
M. stultorum. Brown, Ulus. 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 stria?. 
The Mammiferous Crag, Thorpe, and the Red Crag, Sutton. 
9. Mactra truncata. —The Truncated Mactra, pi. XCI. 
fig. 55. 
M. truncata. Brown, Illust. Rec. Conch. Brit. p. 108, 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, Ayrshiro, and Frith of 
Forth. 
Genus LX.—MACTRINA.— Broicn. 
Shell sub-triangular, equi valve, nearly equilateral; beaks 
