236 ARCADE. 



length ; the triangular area beyond it is concave and nar- 

 row, and the terminal costellse lie rather further apart than 

 the rest. The anterior side is decidedly short, its upper 

 corner is angular (the angle is generally almost a right one), 

 the lower extreme of it is obliquely rounded, the inclination 

 of both anterior and posterior edges being oblique and 

 backward. The hinder termination is biangulated, the 

 uj^per corner being an obtuse, the lower an acute, angle. 

 The ventral margin usually displays but little hiation in 

 the young, wherein it is usually nearly rectilinear, but in 

 the more aged individuals the gape is often of extreme 

 magnitude ; in both the inclination is almost rectilinearly 

 ascending at the longer end. The ligamental area is more 

 or less concave, although projecting a little at the suture : 

 it is of a pale buiF colour, so lineated with broAvn as to form 

 closely disposed obtuse angles at the suture : the actual area 

 occupied by the brownish ligament is small and rhomboidal 

 in outline : in the younger examples it distinctly exhibits 

 an intervening central space, which diminishes in its rela- 

 tive proportion as age advances. The beaks are acute, very 

 far apart, and much inclined. The interior is white, or 

 faintly stained with the external colouring : the teeth of the 

 shorter portion of the hinge-margin are oblique, and rather 

 large ; upon the longer side they are more numerous and 

 crowded, less oblique, and more curved. 



We have examined British specimens of this species 

 alive, and have had the benefit of ample notes on its struc- 

 ture by Mr. Clark and a drawing of it, which we engrave, 

 by Mr. Alder. 



The animal is shaped as the shell, and of a firm sub- 

 stance ; its mantle is open throughout with plain margins, 

 except at the posterior extremity where they are dentated. 

 There are no siphons, but temporary openings can be form- 



