175 



Tocks that yielded this fossil at the three spots must therefore belong to one 

 and the same formation with a very widely extended exposure. 



The figures given by Schnur very well represent the characteristic fea- 

 tures of the Chinese specimens, although they are in no way literally identical 

 with each other. As a whole the Chinese fossils are smaller in size than 

 those from Europe. However, among the European examples, there are 

 also in reality such smaller forms as are comparable with the average 

 forms from China. Other differences are due to the unfavourable condi- 

 tions in which the Chinese fo.ssils had been preserved as well as to 

 the way in which they were collected. Most of the Chinese specimens have 

 not such conspicuous growth lines as are mentioned by Davidson. He says 

 4;hat the growth lines are " numerous close, concentric, regular, imbricating 

 laminae." In the Chinese specimens they must without doubt have been 

 ^worn away. The growth lines are not very clear in the writer's material ; 

 they are somewhat distinctly observable along the marginal portion, but be- 

 'Come more and more indistinct toward the beak. 



In all essential points, however, the diagnosis given by either Schnur, 

 JDavjdson or K.wssr holds equally good for the Chinese species. In this 

 ::paper the description of Kayser is adopted f)r the sake of convenience in 

 oder to explicate the characteristics of the material from China. 



" From circular to transversely oval in outline, mostly wider than long, 

 but also the reverse, with rounded cardinal edges. Two valves almost equal 

 and more or less strongly vaulted, and strongest on the beak (Buckel) where 

 the shell usually attains its greatest height. The greatest width is mostly in 

 the middle. Ventral valve with a comparatively well developed sinus which 

 .becomes significant first at a lif le before the middle of the shell. Corres- 

 ponding to it there is an equally well develop :d fold on the dorsal valve. 

 -Somewhat swollen, moderately long, blunt beak is usually crooked and lies 

 on the beak region of the opposite valve ; penetrated by a l^rge rounded 

 pedicle aperture. Surface witli numerous lamellar, somewhat closely im- 

 -bricating, concentric growth lines." The spirals are shown by polishing the 

 11 ; they are laterally directed. 



