168 Ar t- 6. -M. Yokoyaina 



We possess a right valve which is considerably larger — 36 millim. 

 in diameter — than the specimen figured by Dunker. It is also 

 thicker and deeper (.6 millim. in depth) and may represent a full 

 grown form of the same spicies. 



Fossil occnrrence, — Miyata Zone (Matsubara and Iwaiguchi in 

 Kami-Miyata); Naganuma Zone (Naganuma). Upper Musashino 

 of Shi m osa. 



Living. — Central Japan. 



218. Pectunculus yessoensis, Sowerbt. 

 Pl. XVIII. Figs. 1, 2. 



Pectunculus yessoensis. Sowerbt, Description of Fourteen New Shells from China, Japan 

 and the Andaman Islands, Proc. Zool. Soc. London, 1886. p. 570, pl. XXVIII, fig. 19. Pilsbrt, 

 Cat. Mar. Moll. Jap., p. 150. 



This species is characterized by the marked posterior angle 

 which is, however, not so distinct in young specimens. The 

 surface shows fine decussate lines. Four left valves which we 

 possess are all not full grown. A specimen from Motowada 33 

 millim. high, 36,5 millim. long and 8 millim. deep has the 

 posterior angle rather obscure, while the one from Okine, which is 

 somewhat smaller (30 by 32), is deeper (9 millim,) with the 

 posterior angle more distinct and the hinge line somewhat more 

 bent. 



Fossil occurrence. — Miyata Zone (Motowada and Ôkine). 

 Upper Musashino of Shimosa and Kazusa. 



Living. — Northern Japan (Bay of Hakodate). 



219. Pectunculus nipponicus, Yokoyama. ♦ 



Pl. XVIII. Figs. 3-5, 7 (?). 



Shell rather small, solid, moderately convex, oblique, ovately 

 rounded, inequilateral, anterior side shorter and its margin round- 

 ed, posterior side somewhat produced and either rounded or form- 

 ing a rounded angle at the place where the obliquely descending 

 postero-dorsal margin meets with the ascending postero-ventral 

 margin, ventral margin arcuate. Surface with numerous radiating 



