Fossils from the Upper IMusasbino of Kaznsa and Shimosa. 165 



This shell resembles the preceding in general outline; but 

 it is larger, flatter and smooth on the surface. 

 Fossil occurrence. — Kioroshi. 

 Living. — Eastern seas (according to Adams and Reeve). 



2««. Cmssatella hefet'ofjly^tta, (Pilsbry). 



Crasmtella lieteroi/li/pta. Yokoyama, Foss. Miura Penin., p. 141, pi. XI, figs. 10, 11. 



Crassatellites hetewglypta. Pilsbry, Cat., p. 135. 



Grassatella japonica. Sowerby, Jour. Linn. Soc, XX, p. 399, pi. 25, fig. 19. 



A- single riglit valve. 



Fossil occurrence. — Kioroshi. Lower Musashino of Miyata 

 and Naganuma. 



Living. — Central and Western Japan. 



Family Cyrenidae. 



Genns CîORBICîULA, Mühlfeldt. 



267. Corhictila samlaifortnis, Yokoyama. 



PI. XIII. Figs. 14, 15. 



Shell thick, roundly triangular, about as high as long, a little 

 inequilateral, moderately tumid. Anterior border rounded, 

 posterior subtruncate, forming an obtuse angle with broadl}^ curved 

 ventral border; antero-dorsal meeting with postero-dorsal at. an 

 angle which is nearly a right angle; ventral less arched in posterior 

 half than in anterior. Surface with a very l)lunt posterior carina 

 and ornamented throughout with rude, unequal, concentric 

 grooves. Beaks rather large, swollen, prominent. Lunula lan- 

 ceolate, l)ounded by a fine impressed line on both sides. 



The following are some of the measurements of the normal 

 i'orm : 



Length. Height. Depth. 



Left valve 26.7 millim. 26.5 millim. 9.0 millim. 



28.8 „ 29.3 „ 9.5 „ 



Eight „ 31.5 „ 31.5 „ 9.9 „ 



Besides, there are also forms which are either shorter or 

 longer. One of the latter measures 21 millim. in length, 19.6 

 millim. in height and G. 7 millim. in depth, while one of the former 



