16 G. O. Sårs. 



South Australian species, E. rivolensis, Brady, a form which 

 is still, however, very imperfectly known, and perhaps does 

 not even belong to this genus. 



Seen from the side (PI. II, fig. 1), the shell exhibits a 

 rather regular oblong or elliptical form, and does not show 

 even the slightest trace of any umbones. The greatest 

 height of the shell occurs somewhat in front of the middle, 

 and but little exceeds 2 /a of the length. The dorsal margin 

 appears rather gently curved, and has its greatest convexity 

 a little in front of the middle. It terminates both anteriorly 

 and posteriorly in a well-marked angle, from which the free 

 edges of the valves take their origin. These are evenly 

 curved throughout, though anteriorly and posteriorly their 

 curvature is somewhat stronger than in the middle. Both 

 extremities of the shell appear obtusely rounded and nearly 

 equal, or the anterior one very little broader than the 

 posterior, having also the upper angle somewhat more 

 pronounced. 



Seen from above (fig. 2), the shell is found to 

 be rather compressed, exhibiting a narrow cuneate or 

 somewhat fusiform shape. Its greatest width, which does 

 not attain half the height, occurs rather in front of the 

 middle, and the posterior extremity appears considerably 

 narrower than the anterior. 



The valves are exactly alike, and exhibit a smooth 

 and polished surface, without any .trace of hairs or spines. 

 They each have, however, a number of very delicate con- 

 centric striæ, the so-called lines of growth. These striæ, 

 which lie somewhat closer together in front than posteriorly, 

 do not seem to exceed 4 in number on each valve, and 

 by the innermost a rather large oval area is defined, within 

 which, somewhat anteriorly, the insertion of the adductor 



