GASTROCHiENA. 91 



away in the line of excavation. Several exotic kinds 

 are known. One of these excavates coral, and when 

 full-grown encases itself; bnt the coral soon outgrows, 

 smothers, and completely envelopes the Gasti^ochcena. 

 Instinct seems in this case to be at fault. The Jurassic, 

 cretaceous, and tertiary formations contain many species. 



GastrochtENa du'bia^, Pennant. 



Mya duhia, Penn. Br. Zool. iv. p. 82, t. 44. f. 19. G. modioUna, F. & H. i. 

 p. 132, pi. ii. f. 5-8, and (animal) pi. F. f. 5. 



Body club-shaped, pale brown with a reddish tinge : mantle 

 corrugated : tubes slender and transparent, capable of being 

 extended to three times the length of the shell ; terminal cirri 

 short, dark purphsh-brown : gills narrow, and of a thin tex- 

 ture, finely pectinated on both surfaces : paljiDS smooth without 

 and striated within : foot very expansible, conical when at 

 rest. 



Shell elongated and obliquely twisted from the beaks to 

 the posterior end, so that its proportions and course of growth 

 are those of a Mytilus, convex, rather thin, opaque, and lustre- 

 less : sculpture, distinct but irregular concentric striae, which 

 are slightly imbricated in front : colour whitish : epidermis 

 yellowish-brown : margins narrow and acutely angular at the 

 anterior end, largely excavated in front (exhibiting an oval 

 gape), with a bold sweep towards the posterior side, which is 

 broad and obtusely angular ; dorsal margin on the latter side 

 long and high-shouldered or raised ; anterior dorsal margin 

 extremely short and truncated : healcs blunt and inflected ; 

 ligament semicylindrical, somewhat prominent, yellowish- 

 brown ; posterior end attenuated : hinge-line nearly straight : 

 hinge-plate rather broad, thin, and reflected outwards ; it is 

 thickened within, where it projects downwards, occasionally 

 resembhng a large and blunt triangular tooth; the anterior 

 edge is also callous : inside porcelain-white, glossy, and faintly 

 iridescent : pallial scar usually indistinct : muscular scars irre- 

 gular on the anterior side ; the posterior one is large and 

 triangular. L. 0-8. B. 0*35. 



Var. ovalis. Shell shorter, broader, and thinner. 

 * Doubtful. 



