|42 Al 't- 6— M. Yokoyauia : 



Sowerby described this species as follows: 



,, Shell ovately trigonal, strong, inequilateral, dark yellow 

 with dark radiating lines, beaks pointed, near together; dorsal 

 area moderately excavated on both sides; anterior border roundly 

 arcuate, posterior biangulated, postero-dorsal margin straight and 

 sloping.'" 



To this he adds that the rough irregular striation in place of 

 the ordinary grooves is peculiar. 



We possess a few perfect specimens from Naganuma, one of 

 which is 33 millim. long, 27 millim. high and 17 millim. thick, 

 and a left valve from Ökine 28 millim. long, 23 millim. high, and 

 flatter in comparison to the above, being only 5 millim. deep. The 

 so-called biangular character of the posterior border is caused by an 

 oblique truncation which makes the postero-dorsal angle obtuse 

 and the postero-ventral angle rather sharp. The edge running 

 from the beak to the postero-ventral angle is distinct, though rather 

 blunt. The ventral margin is broadly arcuate with the portion 

 near the posterior margin slightly concave. The inner margin of 

 the shell is finely crenulate, a character strangely not mentioned 

 by Sowerby. 



Fossil occurrence. — Miyata Zone (Nagai); Naganuma Zone 

 (Naganuma). 



Living. — Central and Western Japan. 



179. Crassatella oblongata, Yokoyama. 

 Pl. XI. Figs. 8, 9. 



Shell rather small, moderate in thickness and convexity, 

 somewhat inequilateral, transversely oblong, rounded in front and 

 truncate behind, the truncated end forming nearly a right angle 

 with the broadly arcuate ventral margin, though its corner is 

 blunt. Surface with many, equal, concentric, rounded ribs whose 

 number varies betwen twenty-five and thirty, and which are about 

 as broad as the spaces between; a rounded edge runs from the beak 

 to the postero-ventral corner, with the surface behind it slightly 

 depressed. Lunula longly ovate, bounded by sharp edges. ' Area 



