25 



visceral disc depressed, convex, and the margin all round abruptly curved 

 down for one-third or one-half the whole length of the shell ; sometimes 

 the shell uniformly arched from beak to front ; the umbo often so greatly 

 developed as to overhang the hinge-line and bring the area under the body 

 of the shell ; in other specimens the convexity of the umbo is continued 

 along the middle to the front, producing a broad mesial carination ; in 

 many the front is greatly produced in a gradual slope from the anterior 

 margin of the disc, and occasionally we find specimens with the front 

 margin so much curved as to be to some extent inrolled under the shell ; 

 in all the umbo is more or less prominent, there being a somewhat flat or 

 depressed sub-concave space of greater or less extent on each side 

 extending to the cardinal angles. The dorsal valve is usually not so 

 much curved as the ventral, thus leaving a comparatively large space for 

 the animal. 



Area of the ventral valve from one-fourth of a line to one line in width, 

 flat at the base, concave above, obliquely striated, with the exception of 

 a portion at the extremities varying from one-tenth to one- third the length 

 between the foramen and the cardinal angles. The stv'm at the sides 

 of the deltidium reach nearly to the upper edge of the area, but they 

 gradually become shorter outwards, leaving a smooth space above them. 

 Deltidium flat, or nearly so, triangular' ; width at the base equal to a little 

 less or a little greater than the height ; a small notch in the middle of the 

 lower edge, most probably for the passage of a pedicle ; from this notch 

 a narrow ridge extends upwards to the beak ; cavity of the umbo divided 

 into two' compartments by a rather strong rostral septum. On each side 

 of the septum there is sometimes a small projection, probably represent- 

 ing the bilobed process. 



Area of dorsal valve about half-a-line wide, and not so variable in its 

 dimensions as is that of the ventral. It seems to be not quite flat but 

 slightly convex, and to form an angle of about 90^ with the basal flat por- 

 tion of the area of the ventral valve. 



In the interior of the ventral valve the divaricator scars are large, 

 sub-pyriform, and one-third the length of the whole shell. The occlusors 

 are ovate, half the length of the divaricators, often with the surface covered 

 with minute corrugated wrinkles like the scars of some species of Pro- 

 ductus. The vascular impressions are well marked on some of the casts 

 of the interior, but vary in the number of the branches, usually from 

 three to five in the width of one line at the margin. In thin shelled 

 individuals they are not seen at all. Interior of dorsal valve not observed. 



Surface very variously striated. In some the strice alternate in size, 

 there being one set of fine sharply elevated lines distant from half a line 



