220 



the Chinese specimens with the said figures is sufficient for the identification 

 of the former with Rhynchonclla plenrodon. The only difference noteworthy 

 between them, as has alreadj' been stated, Hes in the diversity in size, the 

 Chinese examples being remarkably smaller. The following verbal descrip- 

 tion may be helpful for understanding the nature of the Chinese specimens. 



The shell is roughh' speaking transversely oval in outline, composed of a 

 very strongl}- convex dorsal \'alve and relatively flat ventral one. The beaks 

 are pointed, especially the ventral one, which extends somewhat beyond the 

 opposite beak. There seems to be a circular pedicle foramen at the attenuated 

 extremity of the ventral beak, although it is not very obvious in the present 

 material. The cardinal area is hardly recognizable. In the umbonal region 

 of the ventral valve there rises along the medial line something like a low 

 ridge which has a ver\- strong curvature and becomes much depressed in the 

 anterior half or 2/3 of the shell. In olher words, the median sinus of the 

 ventral \'alve is recognizable first at a considerable distance from the beak, 

 the shell being raised along the median line within that distance. On the 

 contrary the lateral portion of the ventral valve is concave instead of being 

 convex, and consequently two of the radial ribs between the median sinus 

 and the lateral parts rise very abrupt!}" and conspicuousl)' above the surface 

 of the median sinus. The dorsal valv^e is more or less evenly and strongly 

 convex, with a slightl\- prominent median fold corresponding to the median 

 sinus of the ventral valve. Although the median fold is not very con.spicuous 

 in relation to the sinus, the anterior margin is somewhat rectangularly concave, 

 and the concavity is occupied by the tongue like prolongation of the ventral 

 valve. The surface of botli the valves are ornamented with rather few, 

 prominent, angular, radial ribs, of which three are found in the median sinus 

 of the ventral valve ; on each side of the ventral \'alvc six of them occur. On 

 the median fold of the dorsal valve there are four of them, and on each of the 

 lateral parts there are five. The ribs being very .strongly angular, the margin 

 exhibits a very remarkable zigzag, of which the crests or angles are very 

 acute on the lateral parts in the ventral valve, while they are so on the 

 anterior or the median part in the dorsal valve. 



