WALAC0Z0A. TROPIOrODA. LAMELLIBRAXCIIIATA. 295 



umbo to the posterior-inferior margin ; the cardinal lamina? 

 elliptical, concave, very thin, projecting vertically, and strength- 

 ened by an oblique fold and a narrow plait, running backwards 

 and downwards, with a glossy pearly space near the hinge ; 

 the colour white. The pearly space is more conspicuous in dead 

 shells, in which it retains its lustre, while the rest of the shell 

 has become dull. I have seen a pearl or pearly protuberance 

 formed in it. Length nearly an inch and a-half, height an inch. 



Frequently brought up by the fishing-lines, from deep water, 

 off Aberdeen. Found by me in December, 1841, afterwards 

 by Mr. Leslie, and in October, 1842, by Miss Marion Mac- 

 gillivray. 



Mya praetenuis. Penn. Brit. Zool. Ed. n. iv. 160. PI. 50. f. 1. — 

 Myaprastenuis. Mont. Test. Brit. 41. PI. 1. f. 2. — Auatina prae- 

 tenuis. Turt. Brit. Biv. 48. PI. 4. f. 4. — Amphidesma praetenue. 

 Flem. Brit. Anim. 432. 



2. Anatina truncdta. Truncated Anatina. 



Shell subovate, rather compressed, very thin, brittle, pellucid, 

 with the anterior end much larger and rounded, the posterior 

 narrowed and subtruncate, the anterior dorsal outline little con- 

 vex, the posterior nearly straight ; concentrically rugoso- 

 striate ; the surface slightly glossy, very minutely punctulate, 

 at the posterior end roughish with minute prominences ; an 

 obscure ridge from the umbo to the posterior-inferior margin j 

 the cardinal lamina? oval, concave, thick, projecting vertically, 

 and strengthened by an oblique fold (but no narrow plait) 

 running backwards near the margin, with a glossy pearly 

 space near the hinge ; the colour white. Length of an indi- 

 vidual an inch and a-twelfth, greatest height seven and a-haif 

 twelfths. 



This species differs from Anatina praetenuis in being larger, 

 in having the anterior part higher, the posterior shorter, nar- 

 rower, and more truncate, the cardinal lamina much thicker, 

 without a narrow plait supporting it. 



First found by me in October, 1842, among shells and coral- 

 lines from deep water, off Aberdeen. 



Mya declivis. Mont. Test. Brit. PI. 1. f. 2. — Anatina declivis. 

 Turton, Brit. Biv. 47. — Amphidesma declive. Flem. Brit. Anim. 

 432. — Anatina truncata. Linn. Syst. Ed. 2. vi. 77. 



Genus 6. Thracia. 



Shell ovato-oblong, inequivalve, inequilateral, very 

 thin, brittle, rounded anteriorly, more or less truncate 



