LUCINA. 55 



Cryptodon flexuosum, Turt. (not of Moller, from type) Dithyra Brit. p. 121, 

 pi. 7, f. 9, 10. — Macgilliv. Moll. Aberd. p. 278.— 

 Brown, Illust. Conch. G. B. p. 99, pi. 39, f. 4, 5. 

 Luclna flexuosa, Flem. Brit. Anim. p. 442. 

 Ptychina biplioata, Philippi, Moll. Sicil. vol. i. p. 15, pi. 2, f. 4, and vol. ii. 



p. 11. 

 Lucina flexaosa, Gould, Invert. Massach. p. 71, f. 52. 



„ sinttosa, Brit. Marine Conch, p. 74. 

 Axinus Jiexuosus, Loven, Index Moll. Skandin. p. 38. — King, Ann. Nat. Hist, 

 vol. xviii. p. 242. 



This delicate and fragile bivalve is of a distorted subor- 

 bicular shape, very inequilateral, and manifestly broader 

 than it is long. It is very thin, more or less ventricose, 

 almost pellucid, slightly glossy, and both externally and 

 internally of a pure or bluish-white ; the surface is almost 

 smooth, or is merely concentrically traversed with obsolete 

 subpliciform wrinkles, or lines of growth. The peculiar 

 characteristic features are evidently the somewhat angular- 

 ly flexuous outline, and the posterior sulcus. This latter, 

 emanating from behind the umbones, and running adjacent 

 (but not parallel with, for it inclines more inwards) to the 

 dorsal edge, is rather broad at its termination where it en- 

 larges, and induces a very distinct incurvation of the pos- 

 terior outline towards the middle of the hinder side. Above 

 this there is occasionally also a retusion near the end of the 

 dorsal margin, which slopes with but moderate declination, 

 and trifling convexity. The front dorsal edge slopes re- 

 tusely and very decidedly downward, forming rather more 

 than a right-angle with the greatly rising and little convex 

 anterior part of the ventral margin ; which latter, in the 

 adult shell (for immature individuals are far less angular in 

 outline) appears, owing to its abrupt ascent and similar 

 want of convexity behind, to display subcentrally a round- 

 ed-oif angle ; its posterior rise is, however, less marked, 

 and there is no distinct angle at its cessation. The anterior 



