PlSCIDIlM. 



CONCHIFERA. 



209 



Transversely wedge-shaped ; anterior side considerably 

 angulatcd ; surface ■with numerous fine lines of growth. 



The Plastic Clay, Charlton, Upnor, Plumstead, and New- 

 cross. 



5. Cyrexa obovat.\. — The Egg-ovate Cvroua, pi. 

 LXXXVI. fig. 1. 



Cyc/as ohorata. Sowerby, II. p. 140, pi. 1G2, figs. -I, 5, 6. 



Obovate, length and breadth equal ; gibboso ; anterior side 

 obtuse ; beaks large ; surface with rather strong, irregular 

 lines of growth. 



Upper Marl, Col well Bay, and Barton, 

 f. Cykena pulchra. — The Beautiful Cyrena, pi. 

 LXXXIII. fig. 30. 



Ciiclas ptdcher. Sowerby, W. p. ol, pi. 527, fig. 1. 



Sub-orbicular, convex ; posterior side truncated ; surface 

 smooth ; hinge with one sharp-edged and two bifid teeth ; 

 lateral teeth obtuse and plain ; substance thin and slender. 



The Upper Marl, Hampstead Clitif and Isle of Wight. 



Genus XXXIV.— CYCLAS.—Z?r«^Meire. 



Shell generally sub-orbicular ; ventricose, equivalve, nearly 

 equilateral, transverse, and thin, covered with a delicate oliva- 

 ceous epidermis ; two very minute, divergent, cardinal teeth 

 in both valves, one of which is double in the left one ; lateral 

 tocth two, remote and a little elongated, laminar, compressed, 

 and acute ; and four in the other, two of which are very small, 

 situate on each side of the hinge ; two lateral ovate muscular 

 impressions in both valves, that of the mantle entire, and des- 

 titute of a sinus ; ligament external and slender. 



1. Cyclas media (?.)— The Middle Cyclas, pi. LXXXVI. 

 fig. 14. 



C. medius. Sowerby, p. 51, pi. 527, fig. 2. 



Depressed, thick, transversely obovate ; anterior side small, 

 posterior side somewhat pointed ; surface smooth ; one tooth 

 under the beaks in both valves. 



The Weald, of which it is the characteristic, Kent, Sussex, 

 and Surry. 



2. Cyclas jiEMBnANACEA. — The Membranaceous Cyclas, 

 pi. LXXXVI. fig. 28. 



C. memlranacea. Sowerby, VI. p. 52, pi. 527, fig. 3. 

 Depressed ; very thin ; transversely ovate ; anterior side 

 small ; posterior side a little pointed. 



The Weald, Dorset, Sussex, and AViltshire. 



3. Cyclas ancclata. — The Angular Cyclas, pi. LXXVI. 

 fig. 6. 



C. angulata. Sowerby, Geo. Tr. 2d Ser. IV. pi. 21, f. 12. 



Sub-triangular; beaks prominent; anterior side rounded, 

 jjosterior side truncated ; its line from the beaks to the trun- 

 cation nearly straight ; surface smooth, with shallow lines of 

 growth. 



The Weald, Sussex, the Isle of Purbeck, and South Wilt- 

 shire. 



4. Cyclas scb-«uadrata. — The Half-square Cyclas, pi. 

 LXXXVI. fig. 13. 



C. tub- quad rata. Sowerby, Geo. Tr. 2d Ser. IV. pi. 21, 

 fig. 8. 



Transversely elongated; an oblong square; both sides 



nearly straight ; beaks central and small ; back and base 

 straight and parallel ; surface with strong concentric furrows. 



The Weald Hastings, and St Leonards. 

 .'). Cyclas elongata. — The Elongated Cyclas, pi. 

 LXXXVI. fig. 12. 



C. elongata. Sowerby, Geo. Tr. 2d Ser. IV. pi. 21, fig. .0. 



Transversely elongated ; beaks nearly central ; anterior 

 side rounded, posterior side obliquely truncated, and angular 

 above, rounded beneath ; dorsal and basal lines nearly straight 

 and parallel ; surface smooth, with three or four distinct lines 

 of growth. 



The Weald, Sussex, and the Purbeck Beds, AVhitchurch 

 and Teffont. 



C. Cyclas major.— The (Greater Cyclas, pi. LXXXVI. 

 fig. 7. 



C. Major. Sowerby, Geo. Tr. 2d Ser. IV. pi. 21, fig. 13. 



Sub-rotund ; rather convex ; beaks nearly central, and 

 produced ; one side rounded, the other a little narrow, with 

 a slight angle ; basal line undulous ; surface smooth. 



The AVeald, Kent, Pulborough, Henhurst, &c. 



7. Cy'clas gibbosa. The Gibbous Cyclas, pi. LXXXVI. 

 figs. 8, 9. 



C. gihhosa. Sowerby, Geo. Tr. 2d Ser. IV. pi. 21, fig. 11. 



Slightly ovate transversely ; beaks nearly central, large, 

 and produced ; a concave space under them ; anterior side 

 rounded ; posterior side narrowed, a little truncated, and 

 acute below ; surface smooth. 



The Weald, Sussex, and Purbeck Beds, Whitchurch. 



8. Cyclas parva.— The Small Cyclas, pi. LXXXVI. 

 fig. 16. 



C. parva. Sowerby, Geo. Tr. 2d Ser. IV. pi. 21, fig. 7. 



Sub-orbicular, slightly oblique ; posterior side a little nar- 

 rowed below ; surface smooth. 



The Purbeck Beds, Whitchurch, Quainton, and Chicks- 

 grove. 



9. Cyclas cornea. — The Horny Cyclas, pi. LXXXVI. 

 fig. 20. 



C. cornea. Brown, lllust. Rec. Conch. Brit. p. 93, pi. 39, 

 fig. 19. 



Sub-globose, ventricose ; beaks obtuse, with extremely fine, 

 concentric, nearly obsolete stria;. 



The Pleistocene Marine Formations, Clapton, Clockthorn, 

 Grays, and Stutton. 



10. Cyclas rivicola. — The Eiver Cyclas, pi. LXXXVI. 

 fig. 26. 



C. rieieola. Brown, Rec. Conch. Brit. p. 93, pi. 39, figs. 

 16, 17, 18. 



Transversely ovate, gibbose ; beaks central ; both sides 

 equally rounded ; surface with strong, close, regular, concen- 

 tric stria?. 



The Pleistocene Fresh- water Formation, Felversham and 

 Southend. 



Genus XXXV.— PISCIDIUM.—/y«i/«r. 



Shell equivalve, transverse ; sides unequal, completely clos- 

 in" ; in the right valve one, and in the left two opposite, very 

 small, primary teeth ; behind and before two thin lamellar 



2 X 



