Genus JOUANKETIA, Desmoulins. 



181. »Joua li, net i(t katnakureitsis, Yokoyama. 



PI. VI. Fig. 10. 



A single fractured right valve which, however, agrees 

 exactly with a living shell picked up on the coast of Kamakura. 



Shell small, roughly four-sided in outline, with surface 

 divided into two parts by a mesial groove. The anterior half is 

 very convex with the dorsal margin very short and slightly 

 concave, and the ventral margin steeply ascending, almost twice 

 as long as the dorsal, and at first straight, but excavated near the 

 upper end, so that the front border of the shell becomes pointed, 

 though blunt at apex. The sculpture consists of concentric and 

 radiating riblets. The radiating riblets are unequal, rather distant, 

 about ten in number, altei'nately large and small in the middle 

 portion of the surface and quite absent in a small space immedia- 

 tely bordering the mesial groove. The concentric riblets are close, 

 rounded and going over the radiating ones, giving them a crenate 

 appearance. The margin is also crenate. The dorsal margin of 

 the posterior half of the shell is double the length of that of the 

 anterior half, scarcely convex, sloping, the meeting point with the 

 ascending, someAvhat undulatory ventral margin being subtrun- 

 cate. Margin smooth. 



The fossil specimen is only 4.5millim. in length and 3.6 

 millim. in height, while a recent one from Kamakura is 7.6 millim 

 by 5.2 millim. 



AWied to Jouannetia japonica Y okoy'dimsi (Fossils from Minra 

 Peninsula, p. 105, pi. VII. Fig. 5) from the Lower jMusasliino 

 which, however, shows a coarser sculpture. 



Fossil occurrence. — Shito. 

 Living.— Central Japan. 



