187 



sides by two scarps that descend ver}^ steeply into they sinus ; tliey con- 

 verge posteriorly at a very obtuse angle, where the sinual depression begins. 

 As the beak is erect and pointed, there is a small area under it, although 

 in none of the specimens is it ver}' clearly exhibited. The dorsal valve is a 

 little more convex than the opposite one and has a less prominent small beak. 

 Corresponding to the rectangular tongue of the ventral valve the anterior 

 margin is rectangularl}- up-folded. On the surface of the dorsal valve, there- 

 fore, the median portion is abruptly vaulted above the surrounding portion ; 

 it is visible, however, for a small distance from the anterior margin just as in 

 the case of the depression on the opposite valve. The sides of this median 

 fold make an almost vertical slope toward the lateral portion. At the margin 

 the valve also is deflected vertically downwards, and consequently the margin 

 of the shell has not thin edges, but is, so to say, sorrounded by a vertical 

 wall. 



On the surface of both the valves there is a variable number of radial 

 striae in alternation with interspaces of inferior breadth. In the median sinus 

 or on the fold there are as a rule y-S of the striae, but sometimes more ; on 

 the lateral portion there are about i8 of them. These striae are ver\- thin 

 in the beak region, but gradually widen toward the anterior border. They 

 are not likely to increase in number anteri(n4y in any way. 

 Dimension : — 



Length. 



Width. 



Thickness. 



13 mm. 



T<S mm. 



1 1 mm. 



13 



16 „ 



1 1 



16 ,, 



19 u 





16 „ 





14 „ 



iS „ 



19 M 



II 



l.S „ 



20 „ 



13 



iS ,, 



31 „ 



II „ 



Remarks : — The Chinese specimens, on the whole, are much larger than 

 those from Europe ; this is very obvious when one compares the above mea- 

 surements as well as those given by K.wsek in von Richthofex's China, 

 vol. IV. with those found, for instance, in the paper by Loewk. His largest 

 specimen measures iSmm., 19.5 mm., ;ind 1 1.5 mm. in length, width, and 



