91 



equally and moderately convex, without folds or sinus ; hinge-line shorter 

 than the breadth of the shell ; areas narrow, than in the ventral valve slightly 

 the larger. External surface marked by numerous thread-like radiating 

 striae, which increase in number by the interporation of a finer rib between 

 each pair of the larger ones. The whole surface is likewise traversed by 

 concentric lines." 



This definition, the writer is sure, will be found necessary and sufficient 

 for explicating the Chinese form. The following measurements may suppli- 

 ment it to some extent. 



Length. Width. Thickness. Hinge-hne. 



19 mm. 21 mm. 11 mm. 12 mm. 



Davidson's measurements are ; 



10 lines. I I lines. 5 lines. 



Thus the size and the proportion too are almost coincident in the two 

 forms. The thread-like striae number 3 or 4 in a distance of 1 mm. along the 

 margin. 



Locality : — Lo-ling-po near No-lu-ping, Tung-hu-hsien, I-chang-fu, prov. 

 Hu-pei. 



Accompanied by Poitamcrus borcalis described above. 



Geological Age : — In Britain it was found in„the Wenlock Formation of 

 two different localities. Thus the formation which yielded this fossil must be 

 of the Gotlandian Age. 



