HO .. Arf.6. ;r M. Yokoyama-. - 



kauff in Syst. Conch. Gab. v. Mart. u. Clieinn. XI, pt. 2, Mactra, p. 63, pi. 22. fig. 3,3a. Brauns, 

 G-eol. Envir. Tokio, p. 38, pi, IV, fig. 17. Tokunaga, Foss. Envir. Tokyo, p. 40. " 



Mactra quadrangular is. Deshayes, Proc. Zool. Soc. London, 1853, p. 15, No. 5. Reeve, loo. 

 cit., pi. 1, fig. 3. 



Trigonella veneriformis. Dunker, Index Moll., p. 182. 



We possess two specimens of the left valve which are both 

 ventricose, though somewhat differeing in shape. The one is 

 roundly triangular, while the other is more quadrangular. The 

 former has both ends of the shell more pointed than the latter and 

 corresponds to what Lischke at first called Mactra zonata (loc. cit. 

 II. p. 6) and subsequently reduced to a variety of Mactra venerifor- 

 mis (II. p. 121). Indeed, the form is very variable as shown by 

 hundreds of specimens now living in Tokyo Bay. 



Fossil occurrence: — Yokosuka zone (Otsu); Upper Musashi- 

 no of Musashi and Shi mo sa 



Living. — Central and Western Japan; Northern China. 



Genus JLutrttria, Lamarck. 



133. Lutraria radiata, Yokoyama. 



Pl. VII. Fig. 11. 



Two right valves, the larger measuring 68 millim. in length, 

 33 millim. in height, and 7,5 millim in depth. 



The shell is thin, compressed, transversely elliptical, posteri- 

 orly somewhat dilated, strongly inequilateral the anterior side be- 

 ing somewhat less than one-half its posterior side; anterior as w r ell 

 as posterior end rounded, ventral border broadly arcuate and 

 sharp, dorsal border nearly straight. The surface sculpture consists 

 of small wrinkles covering the concentric lines of growth, and faint 

 radiating, more or less straight striae most distinct in the central 

 and anterior portions, but obsolete in the posterior portion and in 

 the region near the beak. Palliai sinus .very large, linguiform. 

 Ligamental pit triangular with a strong tooth in front of it, while 

 the one behind it is very thin. 



The inner side of the shell still possesses traces of a dark pur- 

 plish colour, especially along the margins. 



