Teeebeatula. MOLLUSC A. ACEPHALA. 369 



Length nearly an inch and a quarter ; breadth one inch, convex, of a black- 

 ish horn-colour ; sides abruptly turned in, and flattened, and marked with a 

 few irregular striae ; front margin a little indented on each side, and project- 

 ing in the middle ; perforation somewhat triangular. 



368. T. aurita. — Regularly ribbed ; the large valve broadest 

 in the middle, semicircular in front, and becoming narrow at the 

 apex. 



Flem. Phil. Zool. ii. 493. t. iv. f. 5 — T. costata, Lowe, Zool. Journ. ii. 

 105. t. v. f. 8, 9.— West coast of Scotland. 

 Length about T %ths, breadth T 2 5 t:hs of an inch ; whitish ; the ribs, from the 

 beak to the margin, the most distinct, rounded, and about eight in number, 

 obsoletely wrinkled concentrically ; the ribs, towards the sides, indistinct ; 

 margin crenulated by the ribs ; the small valve nearly orbicular ; the hinge 

 margin subtruncated, or rather obtusely angular, and having the sides de- 

 pressed ; inner surface punctated ; peduncle short, of numerous unequal and 

 tubular threads attached to a complicated tendino-muscular apparatus, and 

 chiefly to the lower valve. I obtained a small specimen of this species at 

 Ullapool, in Loch Broom. The one, of a larger size, described by Mr Lowe, 

 was found by Mr J. Berkely, at Oban. 



EXTINCT SPECIES. 



* Margin of the valves entire, or slightly icaved. 



1. T. subrotunda Circular, depressed, smooth ; valves regularly and equal- 

 ly convex. — Sower. Min. Conch, t. xv. f. 1, 2 In Chalk. 



2. T. ovata.— Ovate or oblong-ovate, depressed, smooth ; small valve flat- 

 fish, slightly pentangular — Sower. Min. Conch, t. xv. f. 3 — In Green Sand. 



3. T. punctata Oblong, depressed; valves equally convex ; edge straight- 

 ened at the front ; the whole surface finely punctated — Sower. Min. Conch. 

 t. xv. f. 4 — In Lias. 



4. T. carnea. — Depressed, smooth, obtusely five-sided ; front edge short ; 

 valves equally convex, slightly flattened along the middle — Sower. Min. 

 Conch, t. xv. f. 5, 6 — In Chalk, 



5. T. subundata Nearly circular, depressed, smooth ; valves equally gib- 

 bous ; front margin straight or slightly depressed in the middle, with one un- 

 dulation on each side of it — Sower. Min. Conch, t. xv. f, 7 — In Chalk. 



G. T. intermedia Obscurely five-sided, rather depressed, smooth ; larger 



valve most convex ; front margin undulated ; three depressions in the smaller 

 valve, and two in the larger. — Sower. Min. Conch, t. xv. f. 8. — Green Sand. 



7. T. scmiglobosa Nearly circular, gibbous, smooth ; largest valve deepest, 



and uniformly gibbous; front margin undulated, with two risings in the 

 smaller valve Sower. Min. Conch, t. xv. f. 9 — In Chalk. 



8. T. biplicata. — Oblong, gibbous ; beak prominent ; sides rounded ; front 

 straightish when full grown, elevated with two distant large plaits — Soivcr. 

 Min. Conch, t. xc. and t. ccccxxxvii. f. 2, 3 — In Green Sand. 



9. T. digona. — Triangular, oblong, gibbous ; beak prominent ; sides round- 

 ed ;- front either convex or concave ; when old, bounded by two prominent 

 angles, alike in each valve.— Sower. Min. Conch, t. xcvi.— Oolite. 



10. T. ovoides.— Ovate, elongated ; beak prominent ; larger valve gibbous, 

 VOL. I. a a 



