292 MALACOZOA. TROPIOPODA. LAMBLLIBRANCHIATA. 



tlie superabundant water with considerable velocity from both 

 tubes, particularly the longest. 



Mactra Listeri, and ^Nlactra piperata. Gmel. Syst. Nat. 3261. — 

 Venus borealis. Penn. Brit. Zool. iv. 96. — listera compressa. 

 Turt. Brit. Biv. 51. PL 5. f. 1, 2. — Mactra compressa. Mont. Test. 

 Brit. 96. — Lutraria compressa. Lamk. Syst. v. 469; Ed. n. vi. 91. 

 — Lutraria piperata. Lamk. Syst. v. 469; Ed. n. vi. 92. — Mactra 

 Listeri. Penn. Brit. Zool. Ed. n. 194. — Amphidesma compressum. 

 Flem. Brit. Anim. 432. 



Genus 4. Amphidesma. 



Animal oval or oblong, much compressed ; the lobes 

 of the mantle united behind for about a third of their 

 extent, and forming a short siphon-tube, from which 

 emerge the anal and respiratory siphons, the former 

 smaller and cyclindrical, the latter somewhat compressed; 

 the branchial laminae unequal ; the foot very large, com- 

 pressed, bent, obtuse. 



Shell elongated, equivalve, inequilateral, very thin, 

 rounded anteriorly, shorter, narrowed, and more gaping 

 behind, glossy, with a delicate epidermis projecting 

 beyond the margins. Umbones very small, httle pro- 

 minent. Hinge with a dilated oblique, concave promi- 

 nence in each valve, and in one the margins grooved to 

 receive the thickened edges of the other. Ligament 

 double; the internal occupying the cardinal fossa, the 

 external smaU. Muscular impressions rather small, 

 distant. 



1. Amphidesma dlhian. White Amphidesma. 



Shell ovate, compressed, pellucid, very inequilateral, its an- 

 terior portion rounded, and half as long again as the posterior, 

 which is narrower and subangulate, somewhat gaping, and 

 slightly bent ; the umbones very small, obtuse ; the valves 

 very thin, delicately striated concentrically, with the svtrface 

 glossy, very mintttely shagreened ; the inner surface also glossy, 

 very minutely ptmctulate, delicately radiatingiy striidate, and 

 iridescent ; the right valve with a thick concave, oblique, 

 adnate tooth, behind the shallow infraumbonal sinus, a minute 

 erect triangular thin toothlet immediately anterior to the umbo, 

 and on each side an elongated thin laminar plate rising in an 



