166 Art. 6.— M. Yokoyama : 



A water- worn right valve, 9 millim. long, 5 millim. high and 

 2,5 millim. deep. It is dorsally depressed, a character not found 

 in the form described by Pilsery. But its sub trapezoidal shape 

 and other characters leave no doubt of its being the same species. 



Fossil occurrence: — Koshiba Zone (Koshiba). 



Living: — Central Japan. 



215. Area symmetrica, Eeeve. 

 Pl. XVII. Figs. 7, 8. 



Area symmetrica. Eeeve, Conch. Icon., Area, sp. 117, Syst. Conch. Cab. Mart. Chemn., vol. 

 Vni, p. 94, pl. 25, figs. 7, 8. 



The shell is small, solid, generally quadrate in outline, 

 swollen, closed, equivalve, somewhat inequilateral, with a sharp 

 keel running from beak to postero-ventral corner. The surface is 

 ornamented with numerous radiating riblets which are more or 

 less granulated. 



The shape is very variable. The normal forms have the 

 upper and lower margins parallel, but these are those which show 

 the height in the posterior portion less than in the anterior, a 

 character which is especially eminent in our fossil and which 

 makes the postero-ventral corner as if somewhat rostrated, In 

 specimens of this kind, the median dorsal depression is pushed 

 more to the posterior side, and anterior margin is more rounded 

 than in the typical ones. The thickness of the shell is also 

 variable. In recent forms which we have, it is either equal to, or 

 greater than, the height, while in the fossil, it is either equal or 

 less. 



Rather frequent. The largest fossil specimen measures 18 

 millim. in length and 12,5 millim. in height and thickness. 



Fossil occurrence. — Yokosuka Zone (Yokosuka); Naganuma 

 Zone (Naganuma). Upper Musashino of Shimosa. 



Living. — Central and Western Japan; Philippines; Indian 

 Ocean. 



