Fossils from the M iura Peninsula ami its Immediate North. ]Q7 



The shell is transversely elongate and oblong, very in- 

 equilateral, swollen, and more so in the anterior than in the 

 posterior portion. The sculpture consists of concentric ribs and 

 radiating rows of tubercles. The concentric ribs are low and rbof- 

 like. with wide Hat intervals, getting posteriorly unequal and 

 irregular, and looking like coarse incremental lines. The radiat- 

 ing ribs which are not present in the posterior portion of the 

 shell are distant and made up of rows of sharp tubercles on the 

 concentric ribs, which get sharper and spiny in the anterior 

 portion. These ribs situated in the middle portion of the shell 

 leave tubercular impressions on the cast, while those situated in 

 its anterior portion leave none. The impressions of concentric 

 ribs on the internal cast are more prominent than the real ribs on 

 the shell's surface. 



This species is like Pholadomya hesterna Sow. (Wood, Crag 

 Moll., Bivalves, p. 166, pi. XXX, fig. 1) of the English Crag 

 which is, however, more swollen and spiny. 



Fossil occurrence. — Naganuma Zone (Naganuma). 



Family Corbulidae. 



Genus Corbula. Bruguière. 



128. Corbula venusta, Gould. 



Pl. VII. Figs. 4-6. 



Corbula venusta. Gould, Proc. Bost. Soc. Nat. Hist., vol. VUE, p. 25. Otia Conch., p. 164. 

 Schrenck, Moll. d. Ainurl. u. d. Nordjapan. Meeres, p. 583, pl. XXV, figs. 1-4. Dunker, Index 

 Moll., p. 177. Tokonaga, Foss. Env. Tokyo, p. 39, pl. II, fig. 22. 



A small, solid, ovately triangular, rather compressed shell is 

 broader than high, rounded in front and obliquely subtruncate 

 behind, with the ventral border broadly arcuate. The surface is 

 marked by a sharp edge running from the beaks to the postero- 

 ventral angle, and there is a shallow depression running long- 

 itudinally in the middle portion, distinct near the beaks, but 

 indistinct toward the ventral margin. The sculpture consist.- of 

 concentric grooves which are rather irregular and unequal. 

 Length 10 millim., height 7 millim., thickness 4,5 millim. 



