anteriorly, ending in a short projection at the front. In the sinus there is ' a. 

 single rib which begins at a point half way from the beak to the anterior 

 margin. On each side of the sinus there are five radial ribs which are rounded 

 on the top and quite simple ; those at the boundary of the sinus and the lateral 

 portion are especially strong. Interspaces are narrower than the ribs them- 

 selves. Dorsal valve less convex, attached to the ventral by a nearly straight 

 hinge-line which is very slightly inclined toward the caid'nxl extremities. 

 Beak is very obscure, hardly projecting over the hinge-line. The valve is 

 uniformly curved antero-posteriorly. There is a large median fold which 

 strictly corresponds to the sinus of the opposite valve. This fold is very high 

 so that the lateral curvature is by no means uniform. On the fold, along its 

 median line, is a depression corresponding to the intersinual fold or rib of the 

 ventral valve. This median fold of the dorsal valve is separated from the 

 lateral portions by two extraordinarily deep furrows, much more remarkable 

 than those in alternation with the ribs on each side of the fold. There are 

 about five of them on eaeh of the alar portions of the shell. Under a magni- 

 fier there are very fine, undulating growth lines which are concave on the ribs 

 and convex in the intercostal furrows. Besides, there are still finer radial 

 striae on the whole surface of the shell, also visible only with the aid of a 

 magnifying glass. 

 Dimensions : — 



Length. Width. Thickness. Hinge-hne. 



9 mm. lo mm. 5 mm. 8 mm. 



12 „ 10 „ 8 „ 10 ., 



Remarks : — There are only four examples from China now at the 

 writer's hand, two of which, however, are very well preserved, and conse- 

 quently make good material for determinations. The most striking difference 

 between the Chinese and the German forms is of course in the size of the 

 shell. This was considered by Scupin as one of the standards that separate 

 the variety from the type, as already mentioned above. Indeed, in point of 

 magnitude the Chinese examples must be identified with S/). ziczac. The 

 ventral area, however, is Very high in relation to the length of the shell, and 

 also, as already remarked, the sinus is prolonged anteriorly. Moreover, 



