LUTRARIA. 375 



not being peculiarly apparent upon the umbonal region. 

 The valves, which gape most widely at the extremities, 

 and especially at the posterior end, where they are more or 

 less reflected, are of a squalid white (often stained with 

 darker tints from the soil they inhabit), covered with a 

 brownish or dark ash-coloured epidermis which is generally 

 more permanent, wrinkled, and deeper coloured near the 

 posterior extremity. The surface is roughened by coarse 

 wrinkles of growth, which are sometimes almost pliciform 

 along the course of the somewhat obsolete umbonal ridge. 

 The ventral margin, whose middle course is straightish, or 

 even subretuse, rises very considerably at each extremity, 

 and more particularly in front. The anterior side, which 

 occupies but little more than one-fourth of the entire 

 length, and from the sub-attenuation of its well-rounded 

 extremity even a still less portion of the area of the valves, 

 has its upper or dorsal edge, which is at first almost straight 

 and scarcely sloping, and then convex and very moderately 

 declining, united to the anterior outline without any 

 marked angularity. The posterior extremity, which al- 

 though the broader one is still slightly attenuated, is more 

 bluntly rounded than the other, but is equally devoid of 

 angularity. The hinder dorsal edge is much incurved, and 

 upon the whole, declines but slightly, its termination not 

 being greatly below the level of the but moderately promi- 

 nent umbones. The beaks are obtuse and incurved ; there 

 is no depression in front of them, but the dorsal outline be- 

 hind them is a little flattened. The interior is of a pure 

 white. Tn the left hinge a single rather large primary 

 tooth, whose lower surface is broadly grooved, interlocks 

 between the great thin laminar oblique posterior and the 

 smaller bifurcated anterior one of the opposite valve. 

 The dimensions assigned to this species by Montagu are 



