1^4 A.1*^- 1-— M. Yokoyauia-. 



subtraneate, postero-dorsal convex, antero-dorsal straight, ventral 

 broadly arcuate; postero- ventilai corner l)luntly pointed. Surface 

 with a l)lunt edge running from beak to postero-ventral angle, 

 smooth in general, though incremental lines are more or less 

 rough. Tooth of right valve triangular, with a large fossa behind 

 it. Palliai line rather indistinct. Anterior muscular impression 

 elongated and pear-shaped, posterior more rounded, though with 

 a short pointed process above. The interior of the shell is marked 

 with irregularly distributed pits which may be due to tlie formation 

 of pearls. 



In general outline this shell resembles the preceding, but it is 

 thinner, less triangular, and always provided with the pits above 

 mentioned which are absent in the other species described in this 

 paper. 



The following are some of the measurements: 



Length. Height. Depth. 



Bight valve 17.9 millim. 12.6 millim. 5.3 millim. 



17.5 „ 



Left „ 14.3 „ 



15.3 „ 



Fossil occurrence. — Otake (r;u-e), Shisui (do). 



ISS, (Jorhtiln semafettsis, Yokoyama. 

 PI. VI. Figs. 19, 20. 



Shell rather thin, moderately ventricose, transversely oblong, 

 nearly equilateral. Right valve a little longer than left, rounded at 

 both ends, but more sharply so behind, while the left valve has the 

 posterior border obliquely subtruncate. Surface with faint 

 radiating striae near the ventral border. An obtuse edge runs 

 from the beak to the postero-ventral angle, and in the left valve 

 there is a shallow depression running from the Ijeak to the antero- 

 ventral corner. Palliai line indistinct. Anterior muscular 

 impression elongated, posterior more triangular. 



The measurements of three valves are as follows: 



