Thetis. 
CONCHTFERA. 
225 
G en us L V.—CR A SS AT ELL A.— Lamarck. 
Shell thick, equivalve, transverse, inequilateral; external 
surface generally covered with a brown horny epidermis, and 
more or less transversely grooved; one valve provided with 
two strong cuneiform, rugose, cardinal teeth, which are some¬ 
times perpendicularly grooved, and one primary tooth in the 
opposite valve ; lateral teeth a wanting or nearly obsolete, two 
strong oblong depressions, the one on the anterior side of the 
umbo somewhat elongated, and not so well marked as that in 
the posterior side ; two remote, lateral, rather oblong muscu¬ 
lar impressions; ligament internal, attached to a concave pit 
situate on the anterior side of the hinge, this space is divided 
by a rib into two portions, the outer half of the ligament is 
externally visible when the valves are closed. 
1. Crassatella sulcata. —The Furrowed Crassatella, pi. 
XC. fig. 31. 
C. sulcata. Sowerby, IV. p. G2, pi. 34.3, fig. 1. Tellina 
sulcata. Brander, fig. 8.9. 
Ovate, transversely elongated ; anterior side produced, ob¬ 
liquely truncated, and defined by a moderately distinct ridge; 
posterior side rounded ; surface covered with transverse ribs 
with deep intervening furrows, which are but faintly marked 
on the truncated side; beaks rather pointed ; internal edge 
crenated. 
The London Clay, Barton. 
2. Crassatella plicata. —The Plaitod Crassatella, pi. 
XC. fig. 22. 
C. plicata. Sowerby, IV. p. G2, pi. 345, fig. 2. 
Oblong-ovate; anterior side defined by an oblique, obtuse 
ridge, and slightly truncated ; whole surface with numerous 
fine transverse plaits ; margin crenated within. 
The London Clay, Barton. 
3. Crassatella compressa. —The Compressed Crassa¬ 
tella, pi. XC. fig. 3G. 
C. compressa . Lamarck, An. du Mus. G, p. 410, pi. 20, 
fig. 5. Deshayes , Coq. Fos. pi. 3, figs. 8, 9. 
Sub-triangular; anterior side shortest and rounded; pos¬ 
terior side nearly straight from the two-thirds downwards, 
from thence obliquely truncated ; the diagonal ridge termin¬ 
ating in an acute angle ; surface with many transverse narrow 
furrows, which become abruptly angular after passing the ridge ; 
internal margin destitute of crenulations. 
Genus LVI. —TELLIM YA. — Brown. 
Shell equivalve, slightly inequilateral; sub-orbicular; con¬ 
vex ; close all round ; left valve destitute of cardinal teeth, but 
provided with a pretty large hiatus, and two projecting later¬ 
al teeth, each having a groove in its centre; sometimes with 
one or two rather long teeth on the right side; right valve with 
two recurved, prominent teeth, which occupy the vacant space 
below the beak in the opposite valve; muscular impressions 
large and distinct; pallial impressions entire; ligament internal. 
1. Tellimya bub-orbicularis. —The Sub-orbicular Telli- 
mya, pi. XC. figs. 34, 35. 
Kellia suh-orhicularis. Turton, Brit. Bio. p. 57, pi. 11, 
figs. 5, G. 
Sub-orbicular, very convex, thin ; beaks nearly central, and 
slightly inflected ; sides nearly equal and rounded ; basal mar¬ 
gin rather straight, a single tooth in one valve immediately 
under the beak, locking into a double incurved one in the 
other ; with a laminated tooth behind the umbo in each valve. 
The Pleistocene Marino Formation, Largs, and the Coral 
Crag, Sutton. 
Genus LVIL—MONT ACUTA.— Turton. 
Shell oval or oblong, equivalve, inequilateral, mostly closed ; 
hinge with two teeth in each valve, and a cavity between them ; 
destitute of lateral teeth ; ligament internal. 
1. Montacuta sub-striata. —The Sub-striated Montacuta. 
M. sub-striata. Turton, Brit. Biv. p. 59, pi. 11, figs. 10,11. 
Tellimya. Brown, Ill. Rcc. Conch. Brit. p. 107, pi. 40, f. 23. 
Sub-ovate, somewhat oblique, a little inflated, slightly con¬ 
tracted in the middle of the valves ; beaks prominent, straight, 
and not quite central; anterior side large and rounded, the 
other short and narrowed ; surface with nearly obsolete dis¬ 
tant stria?. 
The Coral Crag, Sutton. 
2. Montacuta glabra. —The Smooth Montacuta, pi. XC. 
fig. 32. 
Tellimya glabra . Brown, Ill. Rec. Conch, p. 107, pi. 42, 
figs. 20, 21. 
Elliptical, moderately convex, thin, and smooth ; anterior 
side rounded, and posteriorly sub-truncated ; beaks placed 
considerably to one side; one broad primary tooth in each 
valve ; with a central hiatus. 
The Coral Crag, Sutton. 
3. Montacuta ferruginosa. —The Rusty Montacuta. 
M. ferruginosa. Turton, Brit. Biv. p. 60. Tellimya ellip- 
tica. Brown, 111. Rec. Conch. Brit. p. 10G, pi. 42, figs. 1G, 17- 
Transversely sub-ovato, moderately convex, with obsolete 
concentric wrinkles ; beaks obtuse, sub-central, basal margin 
nearly straight; hinge with two projecting teeth, one of which 
is erect, the other slopes inwards and downward, separated by 
a triangular hiatus. 
The Pleistocene Marine Formation, Ireland. 
Genus LVIII.—THETIS.— Sowerby. 
Shell bivalve, equivalve, sub-equilateral, more or less orbi¬ 
cular and convex ; ligament marginal; hinge with three or 
four acuminated teeth; line of attachment of the mantle (?) 
with a deep sinus, extending nearly to the beak ; muscular 
impressions round, small, and remote from the hinge. 
1. Thetis minor. —The Small Thetis, pi. XCII. f. 3, 4, 5. 
T. minor. Sowerby, YI. p. 21, pi. 513, figs. 5 and G. 
Corbula Icevigata. Ib. p. 14, pi. 209, figs. 1 and 2. \ enus, 
Mantell. Geology of Sussex, p. 73, No. 12. 
Shell gibbose, wider than long; beaks pointed, nearly ap- 
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