Fossils from the Upper Musashino of Kazusa and Shiinosa. J^29 



11.3 millim. in length, 10.2 millim. in height and 3 miUim. in 

 depth. It is coloured pinkish, especially near the beak. 



Fossil occurrence. — Shisui (frequent), Tega. 



Living. — Central Japan. 



19«. Miictm 8acha1iti€Èisis, Schrenck far, ittiperinlis, Yokoyama. 



PI. YII. Fig. 9, 10. 



Mactra sachalinensis. Tokunaga, Foss. Env. Tokyo, p. 39, pk II, fig. 25. 



Shell large, thick, tumid, triangular, anterior side shorter than 

 posterior, rounded at both ends though generally more sharply 

 behind than in front, postero-dorsal slope either straight or only a 

 little arched, ventral margin broadly curved with the anterior half 

 sometimes less curved or even nearly straight, postero-ventral and 

 antero-ventral corners generally indistinctly and obtusel}^ angulate. 

 Surface with very unequal and irregular concentric grooves. 

 Beaks comparatively small, very obtuse, with indistinct edges 

 running both to postero-ventral and antero-ventral corners near 

 which they become more or less distinct. Ligamental pit triangu- 

 lar, protruding downward far out of the hinge-margin. Anterior 

 muscular impression fig-shaped, posterior more squarely oval. 

 Palliai sinus deep, tongue-shaped. 



There is a considerable variation in the shape of the shell, 

 some being more ovate than others. 



The figured specimens are very large ones, if not the largest. 

 The right valve (fig. 10) measures 119 millim. in length, 90.5 

 millim. in height and 33 millim. in depth, while the left (fig. 9) 

 which is more triangular in shape is 111 millim. in length, 86,5 

 millim. in height and 33 millim. in depth. There is also a 

 broken right valve 110 millim. in height. 



Tokunaga identified this shell with the above named species 

 of Schrenck. But there are several well-marked distinctions 

 between the two. The fossil shell is more triangular in outline, 

 with the anterior muscular impression shorter and the ligamental 

 pit larger and more protruding downward. The palliai sinus, 

 too, is perhaps a little deeper (Tokunaga figures it as if pointed. 



