66 



AKT. 2. — S. TOKUNAGA. 



broad without any distinct limit toward the interstices; they 

 number three or more near the central part of the shell, where 

 each rib is broadest at the ventral margin of the shell, being 

 about 3""" in breadth, and again divided into three or more 

 riblets; near the anterior and posterior portion of the shell broad 

 ribs are separated by a very narrow interspace, and a few coarse 

 lines of growth are visible ; in the younger specimens, the fur- 

 rows and ribs are more sharply bordered, the interspace being 

 wider ; ears nearly symmetrical, radially ribbed, the anterior 

 one only slightly notched. Left valve almost flat, only very 

 slightly swollen ; ribs few, eight in number, sharp and keel-like 

 beneath the umbones, but becoming less sharp near the ventral 

 margin ; interspaces broad and smooth, with very fine concentric 

 scaly lines ; ears radiately striated, the anterior one higher and 

 furnished with broad grooves. 



This species which is abundant at ( )ji, and in some Tertiary 

 districts in Japan, was identified by Brauns with Pecten plica L., 

 but according to my own observations, there exist many points 

 of difterence between the two. P. plica L. is a small shell with 

 a very small auricle, nearly equal blunt radial striations, and 

 only three central ribs and two much smaller ones in each 

 valve. These characters alone are sufficient to prove that they 

 do not belong to the same species. 



