10 TESTACEOUS MOLLTJSCA. 



GASTROCH^NA. 



liJquivalve, somewhat wedge-shaped, with a iiery large oval oblique 

 posterior opening between the valves, the anterior extremity 

 nearly closed; hinge linear marginal and without teeth. 



G. CuNEiFORMis. Lam. 1. — Reeve, t. 20./. 4, 5. — Soiv. G. f. 

 4, b.—Pholas Hians. Gm.. 3217.— D. p. 39.— Ch.f. ]G78 to 81.— 

 W. t. 2.f. 11. Oval, thin, seraitransparent, greyish white, with the 

 valves converging above, convex in the middle and wedge-shaped be- 

 low, with transverse arched strise ; hiatus oval and very large, occupy- 

 ing two-thirds of the length : teeth very small. 1^. — W. Indies. 



G. Mytiloides. Lam. 2, — Rump. t. 45. f. P. Oblong, 

 anterior side truncated, and scarcely more than twdce the length of 

 the posterior ; excepting a raised pyramidal radiating anterior 

 space, which is crossed by elevated narrow ribs and covered with a 

 tawny epidermis, the shell is almost smooth. 1^. — Isle of France. 



G. MoDiOLiNA.* Lam. 3.— Sow. G.f. 1, 2, 3.— Reeve, t. 20. f. 

 1, 2, 3. — Mytilus Ambiguus. D. p. 104. — Mya Dubia. Pen. 

 — Don. t. 108.— W. t. 2. f. 23.— G. Hians. Fie. p. 458.— G. 

 Pholadia. Turt. B. t. 2. f. 8, 9. Small, thin, brittle, opaque, 

 rounded at both ends and widely gaping on one side ; pale brown 

 and marked with concentric striae : beaks nearly terminal and 

 rather prominent. — France, burrowed in stone. 



G. Truncata. So7v. Z. p. 1834. Oblong, anteriorly with a 

 rounded off truncation, posteriorly very short, sub -acuminated ; 

 striated and dirty whitish : an anterior thin lamellar epidermis. 

 -jL.. If.. — Panama. 



G. OvATA. Sow. Z. p. 1834. Ovate, whitish, with concen- 

 tric striae, which are slender, lamellar, and following the shape of 



tooth and an internal rib proceeding from the umbo to the basal 

 margin of each valve. Two muscular impressions, the anterior 

 large and obovate, the posterior smaller and situated near the 

 edge of the dorsal margin. 



X. DoRSALis.* Turt. t. 2./. 4, 5.— Sow. G.— Reeve, t. 22. 

 — Flem. p. 455. — Pholas X. Desh in Lam. 6. p. 47. Globular, 

 thin, smooth, greenish yellow, gaping in front, the anterior part of 

 the hinge margin produced ; umbones turgid and sub-callous ; a 

 thickened inner girdle terminated by a tubercle surrounding the 

 gape. This interesting species which forms sinuous perforations 

 in wood is the link betiveen Pholas and Teredo, approximating 

 rather to the former in being without an exterior tube, and posses- 

 sing a shield which is small and divided into two. 



X. Globosa. Sow. Z. P. 1835. Globose anterior dorsal edge 

 sloping, the accessory valves rather large. thj-'T — Valparaiso. 



