ox SOME CRETACEOUS FOSSILS FROM SHIKOKU. 365 



margin. ]jut this difference is probably clue to the mode of preser- 

 vation. 



Very frequent at Tanno ; also occurs at Söyama near Ryöseki, 

 and Yamanokami (iSTagano) near Sakawa. 



Trigonia rotunciata n. sp. 

 PI. XL, Fig. 7, 8, 9. 



Shell subor])icular, slightly broiider than high, somewhat in- 

 equilateral, convex. Beaks approximate, a little pushed anteriorly, 

 prominent, pointed, and incurved, lioth the anterior and posterior 

 margins convex, gradually passing into a less convex ventral margin. 

 Hinge- margin also arched. The escutcheon is not clearly separated 

 from the area, there being only a trace of a broad and flat ridge 

 between, running from the beak to the upper end of the posterior 

 margin, which makes the area slightly depressed along its median 

 line. The other portion of the shell is moderately convex, and sepa- 

 rated from the area by an olrtuse edge, and making with the latter an 

 angle of about 120°. The entire surface of the shell is smooth, if we 

 except a few coarse, shallow concentric sulci near the beak, and coarse, 

 concentric rugce which sometimes appear on the posterior side near 

 the ventral maroin. 



Tn appearance of area and escutcheon this species is very similar 

 to the preceding one. 



Among the f^rms of Trigonia hitherto described, there is none 

 which shows any relation to it. Among the Myophorise, however, 

 there are several corresponding forms which have been mentioned 

 before. 



Like the two foregoing species, Trigonia rotundata occurs mostly 



