Plagiostoma. MOLLUSCA. BIVALVIA. 389 



straightish ; one valve spinous, spines half the length of the shell.— Sower 

 Min. Conch, t. lxxxiii. — In Chalk. 



3. P. punctatum. — Depressed, obliquely ovate, with numerous diverging 

 striae ; anterior side long, straight ; ears nearly equal ; diverging strife trans- 

 versely marked with other very fine striae or points. —Sower. Min. Conch, t. 

 cxiii. f. 1, 2 In Lias. . 



4. P. cardiefor/ne. — Gibbous, nearly circular, longitudinally furrowed, 

 smooth ; anterior side short, straight ; wings equal. — Sower. Min. Conch, 

 t. cxiii. f. 3. — In Oolite. 



5. P. rigidum. — Gibbous, obliquely ovate, with many diverging sharp thread- 

 like ribs, and very minute intervening transverse striae ; anterior side long, 

 straight, very concave ; ears nearly equal. — Soiver. Min. Conch, t. cxiv. f. 1. 

 — In Oolite. 



6. P. ovale — Rather gibbous, oblique, elongated, oval, with many small 

 ribs, and minute intervening transverse striae ; anterior side concave, slightly 

 recurved. — Sower. Min. Conch, t. cxiv. f- 3. — In Oolite. 



7- P. olscurum — Rather gibbous, oblique, ovate, smooth, externally ribbed ? 

 with twenty -five internal sulci ; anterior side flattish ; beaks prominent. — 

 Sower. Min. Conch, t. cxiv. f. 2. — In Oolite. 



8. P. pectinoides — Depressed, a little oblique, obovate, rather angular at 

 the back; beak pointed; surface with twenty or more carinated ribs, trans- 

 versely striated ; internally plain ; margin toothed. — Soiver. Min. Conch, 

 t. cxiv. f. 4. — In Lias. 



9. P. Hoperi.— Transversely ovate, oblique, valves convex, obscurely punc- 

 tato-striated ; striae diverging, deepest upon the sides ; anterior slope straight, 

 concave. — Mant. Suss. 204. t. xxvi. f. 2, 3-15. — Sower. Min. Conch, t. ccclxxx. 

 — In Chalk. 



10. P. ruslicum. — Transversely oblong, oblique, longitudinally sulcated; 

 valves convex ; sulci deep ; ears obscure ; anterior slope straight, convex 

 along the middle. — Sower. Min. Conch, t. ccclxxxi. — In Oolite. 



11. P. Iceviusculum Transversely ^obovate, oblique, convex, longitudinally 



ribbed ; ribs close, broad, irregular, very little elevated, convex ; anterior 

 slope straight, hollow ; ears small, unequal. — Soiver. Min. Conch, t. ccclxxxii. 

 — In Oolite. 



12. P. Brightoniensis — Obovate, depressed, longitudinally costated; pos- 

 terior side eared ; anterior side lunulate, concave, small, acuminated ; mar- 

 gin crenulated — Mant. Suss. cciv. t. xxv. f. 15.— Upper Chalk. 



Gen. GRYPPLEA. — Inequivalve ; larger valve concave ; with 

 a large involutely curved subspiral beak ; lesser valve nearly 

 flat ; hinge a transversely striated pit, with an internal li- 

 gament. 



1. G. incurva. — Elongated, very involute, right side an obscure lobe ; lesser 



valve oblong, externally concave Park. Org. Rem. iii. 209. t. xv. f. 3 



In Lias. 



. 2. G. obliquata. — Oblong, slightly involute, oblique ; right side an obscure 

 lobe ; lesser valve irregularly ovate, externally concave. — Soiver. Min. Conch, 

 t, cxii. f. 3.— In Lias. 



