186 . Art. 6.— M. Yokoyama: 



anterior margin almost straight with its corners rounded. The 

 dorsal and ventral valves are almost equally and regularly convex, 

 without any signs of a longitudinal -fold; beak of ventral valve 

 short, a little curved, obliquely truncated by a foramen margined 

 anteriorly by two small deltidial plates. Surface smooth; shell- 

 structure finely punctate. Length 10 millim., breadth 7 millim., 

 thickness 6 millim. 



The general outline of this shell is like that of Terebratella 

 septata Phil. (Seguenza, Memoria della Societa Italiana di Sciense 

 Naturali vol. I, pi. VII, figs, 1,3) from the Pliocene of Italy and 

 also of Terebratella Mariae Dav. (Monogr. Bee. Brach., pi. XV, 

 fig. 13) living in our seas, but both of these have the ventral valve 

 more convex than the dorsal and also a strongly marked longitudi- 

 nal fold. 



Fossil occurrence. — Koshiba Zone (Koshiba). 



Published March 22nd, 1920. 



