"-158 -^'^t- 1-— M. Yokoyama : 



In shape this shell is much like Montacuta (?) donaclna Wood, 

 var. (Crag Moll., Bivalves, p. 131, pi. XI, fig. 3 a.) from the 

 English Coralline Crag. 



Fossil occurrence. — Shito. Oji in Musashi. 



250. Monfacuta ? yantaliatviii, Yokoyama. 

 PI. IX. Fig. 10. 



A few right valves. 



The shell is very small, rather thin, compressed, transversely^ 

 oblong, with the anterior border more sharply rounded than the 

 posterior, and somewhat inequilateral, .the anterior side being a 

 little shorter than the posterior. The ventral margin is generally 

 only slightly arched or straight or even concave, being as a whole 

 subparallel with the very broadly arched dorsal margin. Surface 

 with very faint radiating lines, only visible with a magnifier. Hinge 

 with two diverging teeth and a triangular vacancy between. 



One of the specimens measures 4.5 millim.' in length, 32 

 millim. in height, and 0.9 millim. in depth, while another measures 

 3.6 millim. in length, 2.7 millim. in height and 0.6 millim. in 

 depth. 



The shell resembles in form the right valve of Kellia suborbi- 

 cularis Mont, figured in Wood's Crag Mollusca (Bivalves, pi. XII, 

 fig. 8 b), though the hinge is quite different. 



Fossil occurrence. — Shisui. 



Genus THYASIKA, Leach. 

 251. Thyasira trif/oviata, Yokoyama. 



PI. IX. Figs. 11, 12. 



Shell small, rather thin, ovately triangular, moderately 

 swollen, very inequilateral, posterior side being about two and a 

 half times as long as anterior, sharply rounded in front, bluntly 

 pointed behind. Antero-dorsal margin concave, postero-dorsal 

 broadly convex, ventral strongly arcuate. Beaks small but con- 

 spicuous, bent forward. Surface with a weak posterior flexure, 

 the furrow being very broad and shallow. The sculpture consists 



