SYNDOSMYA. 321 



S. PRisMATicA, Montagu. 



Much elongated, very inequilateral. 



Plate XVII. fig. 15. 



Ligula prismatica, Mont. Test. Brit. Sup. p. 23, pi. 26, f. 3. 



Mya prismatica, Turt. Conch. Diction, p. 103. — Wood, General Conch, p. 101. 



DiLLW. Recent Shells, vol. i. p. 47. — Index Testaceol. pi. 2, 



Mya, f. 21. 



Amplddesma prismatica. Lam. Anim. s. Vert. (ed. Desh.) vol. vi. p. 128. — Turt. 



Dithyra Brit. p. 52, pi. 5, f. 3.— Macgil. Moll. 



Aberd. p. 294. — Brit. Marine Conch, p. 54 Brown, 



111. Conch. G. B. p. 105, pi. 42, f. 5.— Hanl. Recent 

 Shells, vol. i. p. 42, pi. 2, Mya, f. 21. 

 Syndosmya prismatica, Recluz, Rev. Cuv. Zool. 1843, p. 367; in Chenu, Illus. 

 Conch. Syndos. p. 4. — Loven, Index MolL Skandi- 

 navias. p. 45. 



The iridescent Syndosmya (not that the iridescence is 

 confined to this species) is of an elongated oblong shape, the 

 length more than doubling the breadth ; is extremely 

 inequilateral, much compressed, and gaping slightly at both 

 ends. The texture is very thin and fragile, of a translucent 

 white, which in some specimens is more or less iridescent ; 

 and the surface is polished and almost smooth. The anterior 

 side, which is nearly twice as long as the posterior, has its 

 extremity rounded, but not symmetrically so, as the front 

 dorsal edge which is slightly convex is scarcely or but little 

 sloping, whilst the ventral edge which is arcuated in front, 

 curves more or less obliquely upward to meet it. The 

 hinder portion of the lower margin is more rectilinear, and 

 even indistinctly retuse near its termination, but ascends 

 equally as much as in front, forming a subcentral rounded 

 off acute angle, with the arcuated and moderately sloping 

 dorsal margin. The posterior termination is somewhat 

 bent. The umbonal ridge is almost entirely obsolete : the 

 beaks are small, very slightly inclined forward and but little 



