286 MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 



Saxicava rugosa in form, yet I cannot but consider the two 

 species to be distinct. When much abbreviated, it is Mytilus 

 prascisus of Montagu. 



Frequently brought up by the lines from deep water, and 

 often cast on the beach. 



Solen minutus. Linn. Syst. Nat. i. 1 J 15. — Solen minutus. Mont. 

 Test. Brit. 53. PI. I. f. 4. — Mytilus preecisus. Mont. Test. Brit. 

 ] 65. PI. 4. f. 2.— Hiatella rninuta. Turt. Brit. Biv. 24. PI. 2. f. 

 12. — Solen minutus. Lamk. Syst. v. 453; Ed. 2. vi. 57. — Saxi- 

 cava rhomboides. Desh. Lamk. Syst. Ed. 2. vi. 153. — Hiatella 

 arctica. Lamk. Syst. v. 29 ; Ed. 2. v. 443. — Hiatella arctica. Flem. 

 Brit. Anim. 461. 



Family X. — Mactrina. 



Animal oval or elliptical, compressed, with the mantle 

 adhering toward the borders, closed, open in front for 

 the passage of the foot, and forming behind a single 

 tube, internally double, or two distinct tubes ; a large, 

 bent, compressed, tapering foot ; two distant, large, 

 rounded adductor muscles. 



Shell roundish, oval, or elliptical, equivalve, inequila- 

 teral, more or less gaping at both ends, generally covered 

 with an epidermis ; the hinge always with an oblique 

 triangular depression in each valve, for the internal liga- 

 ment, and a plicate laminar tooth in the left valve ; the 

 external ligament small ; the muscular impressions dis- 

 tant, the posterior roundish. 



Genus 1. Mactra. 



Animal oval, compressed. The mantle with the bor- 

 ders thick and simple, posteriorly forming tw r o united, 

 little elongated tubes. Foot oval, compressed, thin- 

 edged, very long. Branchige small, nearly equal. Ad- 

 ductor muscles distant, strong, roundish. 



Shell equivalve, inequilateral, oval or subtriangular, 

 compressed, concentrically striated ; open a little at 

 both ends, but especially the posterior. Umbones mo- 

 derately prominent. Hinge strong ; the left valve with 

 an elevated, thin, angularly bent tooth under the umbo, 

 leaving a vacant triangular space, behind it a large ob- 



