Possila from the Upper Musashino of Kazusa aud SViimoso. \Q'J 



long). The front dorsal margin is at first somewhat ascending and 

 then very gradually descending, very little arched and going over 

 into the rounded front end. The hinder dorsal margin is sloping, 

 at first straight, but eventually becomes concave, meeting with the 

 convexly ascending posterior margin at about a right angle. The 

 ventral margin is broadly convex, ascending at both ends. The 

 surface is subconcentrically grooved, with grooves narrow, so that 

 the interspaces appear as broad flat ribs. These grooves and ribs, 

 however, are somewhat oblique and not quite parallel with the 

 ventral margin, so that those situated near it end at its posterior 

 half. Teeth numerous, lamellar, outwardly bent in the middle, 

 not quite thirty in number in the anterior and about half as many 

 in the posterior row. Ligamental pit triangular, broader than high, 

 and with a somewhat concave base. lAuiula not developed. Area 

 lanceolate, bounded by a sharp carina on both sides and with a 

 high, elevated, very steep carina in the median line, whose lateral 

 faces show only fine sublongitudinal striations. Anterior muscular 

 impression larger than posterior, ovate in shape; posterior trans- 

 versely oval, bounded within by a strong rounded ridge descending 

 vertically from below the posterior end of the ligamental pit. Sinus 

 large, rounded. The largest specimen is a right valve 36 millim. 

 in length (anterior side 23 millim.), 1(S.6 millim. in height and 4.1 

 millim. in depth. 



Brauns and Tokunaga took this shell for Yoldia lanceolata Sow. 

 (Y. arctica Br. et Sow.) to which, indeed, it has a great resem- 

 blance. Btit the sculpture is ([uite different in the two. In the 

 present species it consists of subconcentric grooves, while in Y. 

 lanceolata of much more o1)lique striae which vanish toward the 

 posterior end of the shell. Moreover, the Japanese fossil is some- 

 what more inequilateral, with the posterior end a little sharper. 



Anyhow it is very interesting that a species much like Yoldia 

 lanceolata was found in the Upper Musashino, as all the living 

 forms of Yoldia are hitherto either arctic or at least boreal, Y. 

 lanceolata itself being i-eported also from Kamchatka and vStrait of 

 Tartary. 



Fossil occurrence. — Shisui (very i-are), Otake (do). Oji. 



