MONTACUTA. 75 



M. bidentata, Montagu. 



Abbreviated, devoid of radiating elevated lines ; front side 

 excessively short, bluntly biangulated. 



Plate XVIII. figs. 6 and 6 a. 



Mya bidentata, Mont. Test. Brit. p. 44. — Linn. Trans, vol. viii. p. 41. — Turt. 

 Conch. Diction, p. 102. — Wood, General Conch, p. 99. — 

 Dillw. Recent Shells, p. 45. 

 Montaeuta bidentata, Turt. Dithyra Brit. p. 60. — Flem. Brit. Anim. p. 465. — 



Macgilliv. Moll. Aberd. p. 302, — Brit. Marine Conch. 



p. 52. f. 58 (badly). — Alder, Cat. Northumb. and 



Durham Moll. p. 95. 

 Petricola „ Gray, Annals of Philos. 1825. — Hanl. Recent Shells, 



p. 54. 

 Erycina „ Recluz, Rev. Zool. 1844, p. 331. 



TeUimya „ Brown, 111. Conch. G. B. p. 107, pi. 44, f. 8, 9. 



In outline this species approaches the subrhombic oval, 

 the dorsal margin running- in some degree parallel with the 

 ventral. The valves are excessively inequilateral, their 

 convexity is but moderate, their substance very thin and 

 fragile, but not at all translucent, and their surface nearly 

 smooth, or at most but striolated, and equally devoid of 

 lustre as of colour. The ventral margin varies from simply 

 convex to subarcuated ; and the hinder dorsal edge, which 

 is elongated, and at first straight or scarcely convex and 

 hardly declining, forms by its junction with it a well- 

 rounded and rather broad posterior extremity. The front 

 side, which is infinitely the smaller, varies, however, in its 

 proportion to the hinder, being at times only one-fifth as 

 large as the latter, whilst, in certain specimens, it occupies 

 more than one- fourth the entire length of the shell. The 

 front dorsal edge is rather retuse than otherwise, and by its 

 very abrupt descent narrows the anterior extremity, which 

 l»v the ordinary absence of arcuation in the front margin 

 appears very bluntly biangulated. The beaks arc acute, 



