Charlie:
 
Plate 49 , no. 4 in T. Kira's book is Plicatula plicata, described as follows:
 
"This has a larger shell than the preceding three species. (P. simplex, P.
australis, and P. muricata)  It is flat, trigonal and well extends toward the
ventral margin.  Tha surface is sculptured by a few, rather broad
divaricating, low, radial ribs and greenish white in color. The interior is
pearly, with the slightly rippled margin. Distribution: southern Honshuand
southwards, below the low tide level."
 
Mary Graham
Wilton, NH