570 Karl A. Grönwall. 



The ventral valve is greatly convex with rounded rectangular 

 outline and a straight hinge-line, across which projects the short 

 and broad, but strongly bent beak. The extreme point of the beak 

 has been corroded away, so that it is not possible to give its appear- 

 ance with certainty, nor to be quite sure as to the structure of the 

 cardinal area. However, there are definite indications that the beak 

 projected only inconsiderably backwards beyond the hinge-line; and 

 the height of the cardinal area has been rather inconsiderable, or 

 about 4 mm. The area increases gradually in height towards the 

 middle of the shell, and forms an obtuse angle with the mesial plane 

 of the shell. It is provided with a double sculpture, consisting 

 partly of fine, horizontal striæ of growth, and partly of coarser, 

 irregular vertical lines which almost give the impression of wrinkles. 

 The beak occupies about Чг of the length of the hinge-line. The 

 hinge-line is long and straight, about -Vio of the greatest width of 

 the valve, and passes, with evenly rounded corners into the lateral 

 contours. The lateral portions of the valve slope fairly abruptly, 

 forming, thereby, small ears, which are not clearly distinguished 

 from the main part of the valve. The anterior margin of the valve 

 forms a circular line. The width of the valve is somewhat greater 

 than its length. 



From the point of the beak to the anterior margin of the valve 

 runs a narrow, grooved sinus, embracing about 3 of the ribs of the 

 valve, and which only gradually increases in breadth and also be- 

 comes somewhat flatter; the sinus causes some little inflexion of the 

 anterior margin. 



The valve is ornamented with close-lying ribs, pretty flat, of 

 varjång breadth, and with a distinct groove between them which 

 diminish in intensity towards the flanks, and also by exceedingly 

 fine, straight striæ of growth. On each side of the sinus there can 

 be counted 40—50 ribs; in the umbonal region the ribs lie more 

 closely together and are fewer in number, and some few dichotomies 

 are observable there. 



The interior of the ventral valve shows a pair of well developed 

 but relatively thin dental plates which descend almost direct down 

 towards the anterior margin of the valve to about ^ й of its length. 

 The dental plates diverge considerably from the wall of the shell 

 towards the interior, as is clearly seen by Fig. 19, where the dental 

 plate to the left is seen to be considerably more distant from the 

 mesial line of the valve, the reason being that that part of the fossil 

 which is visible to the left, has been less corroded and dissolved 

 than that to the right. 



Dimensions: Length about 43 mm., width about 45 mm., length 



