116 CYCLADIDiE. 



p. 37. — Hanley, Recent Shells, vol. i. p. 89. — Gras, Moll. 



T. et Fl. de la France, p. 73, pi. 6, f. 5.— Philippi, Moll. 



Sicil. vol. ii. p. 214. 

 Cardium lacustre, Mont. (1803) Test. Brit. p. 89. 

 TelUyia lacustris, Linn. Trans, vol. viii. p. 60. — Turt. Conch. Diction, p. 180. — 



Wood, General Conch, p. 197, pi. 47, f. 5. — Dillw. Recent 



Shells, vol. i. p. 104, 

 Cydas lacustris, Turt. Dithyra Brit., p. 249, pi. 1 1, f. 1 8. — Flem. Brit. Anim. p. 



453. — Alder, Trans. Nat. Hist. Soc. Newcastle, vol. i. p. 



40.— Gray, Manual L. and F. W. Shells, p. 281, pi. 1, f. 3. 



Brown, Illust. Conch. G. B. p. 94, pi. 39, f. 20. 



This the most delicate and fragile of our Cyclades is of 

 a slightly rounded subrhombic shape, almost smooth (at 

 least, devoid of all regular striulae), diaphanous, and scarcely 

 inequilateral. Except upon the umbonal region, where the 

 shell is moderately ventricose, the valves are compressed, 

 not merely below, but also more or less on either side of the 

 beaks. The surface is lustrous, and of a greyish ash- 

 colour, and occasionally zoned also with yellow at the ven- 

 tral margin, which latter is moderately arcuated, and rises 

 rather the more anteriorly. The umbones, which lean a 

 little forward, are narrow, very projecting, and as it were 

 capped (their surface becoming suddenly elevated above 

 the remaining area, as if surmounted by another pair of 

 younger valves), furnish the peculiar feature by which the 

 species may most readily be distinguished from its British 

 congeners. The margin on either side of them is nearly 

 equally elevated, declines but little, especially behind, and 

 is scarcely convex. Both extremities are broad, and not 

 very uneqvially ; so the posterior which (especially in the 

 young) is more or less sabbiangulated, rather exceeds in 

 width the rounded extremity of the slightly shorter anterior 

 side. The ligament Is small, inconspicuous, not elevated 

 above the dorsal surface, and usually almost colourless. 



We have described the smaller and more ordinary form, 

 but a much more produced angulated and compressed 





