215 



lesser elevation is gradually formed in the dorsal valve, and a corresponding 

 sinus in the ventral one."' This sentence suggests that the sinus or the fold 

 of the shell is not necessarily discovered in young, small examples of the 

 species. Sommer s material seems a very small one, for he describes it as 

 4 mm. by 9 mm. in size. Of this sjaecimen he says, " Sinus und Sat'el fehlen 

 voUig."" It is therefore not strange that in all of the Chine>e fossils sinus 

 and fold are very obscurely developed or even absent ; for the\' are all rather 

 small examples. 



It is not, however, clear why the specimens from the province ol llu-nan 

 are all so smad ; they all belong to one and the same stage of growth. With 

 respect to tl.is point the writer suspects that these fossils may represent a 

 variety of the species. Among all examples ever described, however, there 

 are many that are of nearly the same size, and all of them are included in the 

 specific name of Athyris royssii. The writer, therefore, does not try to 

 separate the Chinese forms as a variety of the species. 



The diagnosis states : The shell is rather small, generally transversely 

 «blong in outline, but sometimes circular, composed of two almost equally 

 convex, valves. The ventral beak is small and somewhat pointed, incurved, 

 truncated by a circular pedicle opening of varying magnitude ; this hole is 

 contiguous with the beak of the opposite valve. In most of the specimens 

 there is hardh' a median sinus that is distinctly reci^gnizable, although in some 

 of the larger examples the shell is somewhat flattened along the median line. 



The dorsal beak is obtuse, only slightly pointed, and croi^ked in beneath 

 the opposite one. The mesial fold corresponding to the sinus of the ventral 

 valve is scarcely developed at all ; in some smaller specimens the shell is 

 ^even very slightly flattened medially. The margin of the shell is prolonged 

 as a lamella, the two valves being already in C()ntact inside the very margins. 

 The surface is ornamented with numerous concentric lines of growth \vhich 

 are somewhat lamellar or scaly. The borders of these lamella: are likely to 

 have been provided with spines, as it is shown in a few of the specimens by 

 radial ribs with rounded tops. The hinge-line is inferior to the greatest 

 breadth of the shell, and not straight but slighU}' slopes down laterall)'. In 



