öS AKT. 2. S. TOKUNAGA. 



Hokkaido, Hakodate, Enôshima (Sagami), Awa, Miyajima (Aki), 

 Loochoo Is., Philippines and Kingsmill Is. 



144. ARCA TENUIS Tok. 

 PI. IV. Figs. 1 a and b. 



This species which greatly resembles A. infiata Rve., has 

 hitherto been taken for the fnll-grown form of A. suhcrenata 

 Lke. I)Ut the fnll-grown specimens of Area tenuis Tok. show 

 both valves quite equal in shape and with no marginal crenula- 

 tions caused by ribs. The distinctions from A. inflaia Rve. 

 are the fol low in o- : 



1. The shell is more pointed at the antero-dorsal edge. 



2. The shell is thinner. 



3. The umbones are wider apart and not abruptly pointed 

 as in A. inflata Kve.; sometimes they are slightly depressed at 

 the middle. 



4. The shell surface slopes towards the posterior edge, 

 gradually and not suddenly. 



5. The breadth of the shell is generally less than in A. 

 inflaia Kve. 



6. The ligamental area is decidedly narrower. 



