572 I^RL A. Grönwall. 



The dimensions of the three specimens are as follows: 

 I. width 34 mm., length 23 mm.: width : length = 100:68 

 II. — 32 — — 26 — — : — = 100:81 



in. _ 40 — — 33 - — : — = 100:82 



The description given below refers to specimen I, and the varia- 

 tions from this form presented by the other specimens, are then 

 given separately. 



The outline of the shell forms, approximately, a parallel trapez- 

 ium. The ventral valve is evenly arched, with the beak rapidly 

 tapering off, with an involute, blunted point projecting beyond 

 the hinge-line. The hinge-line is equal to the greatest width of the 

 shell, and from the angles of the shell the outline goes in an even, 

 flattened arc to the anterior margin of the shell, where the sinus 

 forms a shallow inflexion. In the umbonal region the sinus is deep 

 and its margins well marked ; it increases in width anteriorly and, 

 at the same time, becomes shallower, so that it passes more and 

 more into the lateral portions. The cardinal area is moderately 

 high with almost the same height in its whole width, and is limited 

 towards the shell by a clearly marked elevated ridge. Towards the 

 interior of the shell the edge of the area is bent upwards, so that 

 the area forms a groove. It has possessed an evident horizontal 

 sculpture of fine striæ, the greater part of which, however, disap- 

 peared on working out; it is uncertain if any vertical sculpture 

 existed; on the inner side of the edge of the area there is visible a 

 faint crenulation. 



The shell is ornamented with rounded ribs of almost uniform 

 size; in the umbonal region there are seen only 3 ribs on each side 

 of the sinus and, on the anterior margin there can be seen 8, which 

 gradually vanish towards the sides. The sinus, in which are visible 

 3 ribs of almost equal size, is separated from the lateral parts by 

 a specially prominent rib. On the ribs there is seen a very fine 

 radial sculpture which, together with the faintly undulating striæ of 

 growth, form an imbricate sculpture as shown by Fig. 4. The faint 

 striæ of growth are, in certain places on the anterior part of the 

 shell, bunched together, becoming thereby more prominent, but on 

 the posterior part too of the shell they can be clearly seen when 

 magnified, so that it is possible to follow the growth of the shell. 

 It is then clearly seen that, with the growth due to age, a certain 

 definite alteration of form of the shell has taken place, so that the 

 shell, in the younger stages, has pointed wings, and considerably 

 less length than the grown shell. From the striæ of growth it can 

 be determined that the proportion of length of shell to the width 

 of the hinge-line should be as follows: 



