~292 MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 



the superabundant water with considerable velocity from both 

 tubes, particularly the longest. 



Mactra Listeri, and Mactra piperata. Gmel. Syst. Nat. 3261. 

 Venus borealis. Penn. Brit. Zool. iv. 96. Listera compressa. 

 Turt. Brit. Biv. 51. PI. 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, little 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 small. Muscular impressions rather small, 

 distant. 



1. Amphidesma album. 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 surface 

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

 very minutely punctulate, delicately radiatingly striulate, 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 



