PSAMMOBIA. MOLLUSCA. PSAMMOBIADiE. 437 
valve shallow ; internally a ridge from the beak to the anteal muscular im- 
pression. 
EXTIKCT SPECIES. 
1. T. ohliqm. — Nearly orbicular, convex, oblique, smooth; anterior side 
slightly defined by a longitudinal vfa.ve.-— Sower. Min. Conch, t. clxi. f. 1. — 
Crag. 
2. T. oeata. — Ovate, convex, smooth ; equilateral, with a slight retral 
vf 2 i\e,~-mSower. Min. Conch, t. clxi. f. 2. — Crag. 
3. T. obitisa . — Transversely ovate, convex, concentrically striated ; margin 
obtuse ; sides very unequal— (yotcer. Min. Conch, t. clxxix. f. 4.— In Crag. 
4. T. Branderi. — Obovate, depressed, smooth, with a small sinus on the 
anterior margin ; beak rather prominent. — (T. bimaculata, Brander^ f. 102.)— 
Sower. Min. Conch, t. ccccii. f. \.~-Lond4)H Clay. 
5. T.//o5a— Obovate, depressed, transversely striated ; posterior side an- 
gular; anterior rounded; striae elevated, acute, numerous, enlarged poste- 
riorly.— .S'oecer. Min. Conch, t. ccccii. f. 2. — Londm Clay. 
6. T. amhigua. — Oval, elongated, convex, obscurely sulcated; left valve 
thickest, curved, and with only one huge tooth. — Sower. Min. Conch, cccciii. 
.^London Clay. 
7. T. striatula — Transversely elliptico-lanceolate, convex, smooth ; poste- 
rior part shortest, longitudinally striate ; striae obscure.— Min. Conch, 
t. cccclvi. f. 1. — ^een ^and. 
8. T. inequalis. — Oval, convex, smooth ; posterior extremity obtuse ; its 
surface finely striated from the beak; anterior side largest, rounded.— fibecer. 
Min. Conch, t. cccclvi. f. 2. — Green Sand. 
Gen. CXXII. — PSAMMOBIA. — ^Shell transverse, oblong, ^or 
oval, a little gaping, with a flexure from the beak to the 
anal angle ; hinge with two teeth in one or both the valves ; 
lateral teeth none. 
^ With two teeth in one of the valves only. 
490. V.Jlorida. — Oval-oblong, with close-set transverse striae, 
and minute longitudinal ones. 
Turt Biv. Brit. 86. t. vi. f. 9.— English and Irish coasts. 
Length half an inch, breadth an inch, rather convex ; variously marked 
with different colours ; sloping gently from the beaks ; nearly equally round- 
ed at both extremities ; a single cloven tooth in one valve, and a cavity on 
each side for the reception of the two teeth of the opposite valve. 
491. P. costulata. — Oval-oblong, with transverse striae, and 
minute longitudinal ones, and oblique longitudinal ribs at the 
posterior end. 
Turt. Biv. Brit. 87. t. vi. f. 8 Torbay. 
Length half an inch, breadth an inch; variously coloured ; dorsal margin 
nearly even ; slightly truncated obliquely on the retral extremity, where 
