MACTRA. 351 



fill for tlieir size ; some exotic species attain considerable 

 dimensions. More than fifty species of this genus are 

 clearly ascertained ; they are found in all seas, but the 

 most beautiful are tropical forms. Few fossil examples 

 occur, except in tertiary strata. 



M. soLiDA, Linnseus. 



Oval or subtriangular, equilateral, or almost so, solid, more or 

 less rounded at both extremities, smooth ; dorsal areas grooved 

 concentrically, not much depressed : umbones not greatly pro- 

 minent ; ventral edge not strongly arcuated : lateral teeth 

 grooved. 



Plate XXII. figs. 1, 5, and (siphons) Plate L. f. 2. 



Macira solida, Linn. Syst. Nat. ed. 12, p. 1126. — Penn. Brit. Zool. ed. 4, vol. 

 iv. p. 92, pi. 51, f. 43 A, and pi. .52, f. 43.— Pulteney, 

 Dorset, p. 31.— Donov. Brit. Shells, vol. ii. pi. 61, f. I, 3, 4, 

 5. — Mont. Test. Brit. p. 92. — Linn. Trans, vol. viii. p. 70. — 

 Dorset Catal. p. 32, pi. 12, f. 1.— Turt. Conch. Diction, p. 81. 

 — Turt. Dithyra Brit. p. 67. — Flem. Brit. Anim. p. 426. — 

 Macgil. Moll. Aberd. p. 288. — Brit. Marine Conch, p. 46. — 

 Brown, 111. Conch. G. B. p. 108, pi. 41, f. 3, 4.— Chemn. 

 Conch. Cab. vol. vi. p. 230, pi. 23, f. 229.— Spengl. Skriv. 

 Naturh. Selskab. vol. v. part 2, p. 113. — Dillw. Recent Shells, 

 vol. i. p. 140. — Lam. Anim. s. Vert. (ed. Desh.) vol. vi. p. 104. 

 —Index Testae, pi. 6, Mactra, f. 21.— Phil. Moll. Sicil. vol. 

 i. p. 1 1, and vol. ii. p. 10. — Haxl. Recent Shells, p. 31. 



Trigonella zonaria. Da Costa, Brit. Zool. p. 197, pi. 15, f. I. 

 „ gallina. Da Costa, Brit. Zool. p. 199, pi. 14, f. 6. 



Mactra truncata, Turt. (not Mont.) Dithyra Brit. p. 68 (from type). 



Mactru dubia and M. oralis, Sow. Min. Conch, pi. 160. 



The shape of the species which we are about to describe 

 is very variable, ranging from simply oval to subtriangular. 

 It is solid, opaque, subequilateral, glossy, and of an uniform 

 white under the ordinarily dull yellowish ash-coloured epi- 

 dermis, which veils its outer surface. This latter is almost 

 smooth throughout, except occasionally a few scattered 

 antiquated or obsolete concentric furrows ; the dorsal areas 



