118 Art. 6.— M. Yokoyama: 



than the latter and somewhat arched. Ventral margin broadly 

 arcuate, anteriorly gradually passing into the rounded anterior 

 margin. Posterior fold of the right valve situated near the 

 posterior margin, distinct but not very strong. An obtuse ridge 

 runs from the beak to the postero-ventral notch, forming a narrow 

 area behind, with an obtuse rib running longitudinally in it. 

 The sculpture consists only of unequal incremental lines which 

 are especially marked on the posterior ridge and rib. Beaks small, 

 pointed. Main teeth two, the anterior in the left valve and the 

 posterior in the right valve being bifid. Lateral teeth indistinct 

 Palliai sinus large and deep, much deeper and higher in the left 

 valve than in the right. Three right valves and one left valve. 

 The left valve measures 19 millim. in length 13 millim. in height 

 and 3 millim. in depth, while the largest right valve 

 measures 26 millim. in length, 18 millim. in height and 5 millim. 

 in depth. 



This species is very closely allied to Macoma truncata Zonas 

 (Philippi's Abbild., I. p. 71, 7, N. 2, pi. 1, fig. 2 and Syst. 

 Conch. Cab. X. pi. 4, p. 248, pi. 47, figs. 4-6) which is living 

 near Tokyo. But so far as we can see from a few specimens 

 which we possess, the shell of the present species is somewhat 

 smaller, less trigonal, the posterior part of the ventral margin 

 somewhat more arcuate and the beaks more pointed than in Jonas 

 species. 



Fossil occurrence. — Miyata Zone (Motowada and Nagai); 

 Upper Musashino of Musashi and Kazusa. 



Living. — Northern Japan (Bay of Hakodate). Tokunaga 

 mentions Tokyo, Nagasaki, and Yokohama as the habitat. But 

 this assertion has not yet been confirmed, while Hakodate is 

 certain, as there is a specimen fron] that place in the zoological 

 museum of our University. 



