PsAMMOBiA. MOLLUSCA. PSAMMOBIADiE. 437 



valve shallow ; internally a ridge from the beak to the anteal muscular im- 

 pression. 



EXTIKCT SPECIES. 



1 . T. obliqua Nearly orbicular, convex, oblique, smooth ; anterior side 



slightly defined by a longitudinal wave. — Sower. Min. Conch, t. clxi. f. 1. — 

 Crag. 



2. T. ovata. — Ovate, convex, smooth ; equilateral, with a slight retral 

 wave — Sower. Min. Conch, t. clxi. f. 2 — Crag, 



3. T. obtusa Transversely ovate, convex, concentrically striated ; margin 



obtuse ; sides very unequal. — Soioer. 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. I.— London Clay. 



5. T. ^Vosa.— Obovate, depressed, transversely striated ; posterior side an- 

 gular; anterior ronnded; striae elevated, acute, numerous, enlarged poste- 

 riorly — Sower. Min. Conch, t. ccccii. f. 2. — Londoji Clay. 



6. T. ambigua.— 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 ohscnxe.— Sower. Min. Conch. 

 t. cccclvi. f. 1. — Green Sand. 



8. T. inequalis. — Oval, convex, smooth ; posterior extremity obtuse ; its 

 surface finely striated from the beak ; anterior side largest, rounded. — Sower. 

 Min. Conch, t. cccclvi. f. 2. — Green Sand. 



Gen. CXXII.— PSAMMOBIA.— SheU 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. F.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, wli'ere 



