170 Art. 6. — M. Yokoyauia : 



220. Pectunculus pilsbryi, Yokoyama. 

 Pl. XVIII. Fig. 8. 



Shell small, moderately solid, flatly convex, suborbicular, 

 nearly equilateral, slightly longer than high, with anterior margin 

 rounded and posterior subangulated in the middle. Surface with 

 numerous, unequal, radiating riblets crossed by fine concentric 

 impressed lines so that the riblets appear finely granulated. A 

 fine interstitial riblet is also often present. Beaks small, pointed. 

 Area very narrow, furnished with a few diverging lines. Teeth 

 about twenty, vertical in the middle part of the hinge-line, 

 oblique towards both sides. Inner margin crenulate. 



A few specimens. The largest, which is a left valve, measures 

 20 millim. in height, 22 millim. in length and b,5 millim. 

 in depth. 



Fossil occurrence. — Miyata Zone (Mukaibatake in Shimo- 

 Miyata, and Kami-Miyata). Upper Musashino of Kazusa. 



Living. — Central Japan (Böshü). 



Family Parallelodontidae. 



Genus JParallelodon, Meek et Worthen. 



221. Parallelodon obliquatus, Yokoyama. 



Pl. XVII. Fig. 6. Pl. XVIII. Figs. 9-11. 



Shell moderately thick, convex, equivalve, transversely and 

 longly ovate with the narrower end directed anteriorly, strongly 

 inequilateral, with the posterior side three times as long as the 

 anterior; anterior margin rounded, posterior arcuate with the round- 

 ed postero-ventral corner somewhat produced; ventral margin 

 straight or sometimes even slightly excavated. Surface somewhat 

 depressed in the middle, radiately ribbed ; ribs numerous, fine, 

 unequal, a little flexuous, occasionally with a finer interstitial 

 riblet which becomes obsolete towards the beak, crossed by incre- 

 mental lines and a few concentric grooves. Beaks small, almost 

 touching. Area narrow, with a few sharp diverging ridges. Den- 

 tition consists generally of three subhorizontal, parallel, transverse- 



