215 Art. 6. — M. Yokoyama : 



as well as into the broadly arcuate ventral margin which in the 

 posterior part becomes nearly straight and forms a rounded 

 right angle with the truncate end. Postero-dorsal margin straight, 

 sloping, but somewhat more steeply than the antero-dorsal. 

 Surface with an obtuse edge or keel running from beak to 

 postero-ventral corner, the narrow space bounded by this edge and 

 the postero-dorsal margin being quite flat. The sculpture consists 

 of fine concentric furrows which bend abruptly upward at the 

 edge. Main teeth two, the anterior one thick and bifid, the 

 posterior thin and longer; laterals in the left valve indistinct. 

 Palliai sinus large and deep, approaching to the anterior muscular 

 impression. A single left valve, 10 millim. in length, 5 millim. 

 in height and 1,2 millim. in depth. 



This species may possibly be a Macoma, for it is related to 

 Macoma praemitis Rom (Syst. Conch. Cab., vol. X, p. 257, pi. 48, 

 figs. 7-9) of our seas, but is longer and more inequilateral. It is 

 also like Tellina donacina L. (Forbes a Hanley, Brit. Moll., I. p. 

 292, pi. XX, figs. 3,4) of the Atlantic, in which, however, the 

 anterior side is wedge-shaped. 



Fossil occurrence. — Miyata Zone (Kami-Miyata). Upper 

 Musashino of Musashi, Shimosa, and Kazusa. 



Penus Macotna. Leach. 



142. Macoma dissimilis, (Mabtens). 



Pl. VII. Figs. 19, 20. 



Macoma dissimili*. Pilsbry, Cat. Mar, Moll. Jajan, p. 125. 



Macoma nasuta. Fokunaga, Foss. Env. Tokyo, p. 45, pl. Ill, fig. 2. 



Tellina dissimilis. Martens, Ann. Mag. Nat. Hist., Series in, vol. 16, p. 430. 



Tellina nasuta var. dissimihs. Lischke, Jap. Meeresecnch., II, p. 115, pl. IX, figs. 15-17. 



'Tellina nasuta. Brauns, Geol. Env. Tokio, p. 39. 



Without going into the discussion of whether Macoma 

 dissimilis Mart, is identical with Macoma nasuta Conrad or not, I 

 simply say that two right valves with a very strong posterior fold 

 agree quite well with the shell described by Lischke as a variety 

 of Macoma nasuta in his work on our marine mollusks. The so- 



