Fossils from the Miura Peninsula and its Immediate North. \QQ 



impressed lines crossed by concentric lines of growth, with the 

 points of intersection sometimes looking like minute pits. Inner 

 surface smooth, with only irregular radiating lines most distinct 

 to wards the margin which is finely crenulate. Beaks small but 

 pointed, not touching. Area distinct, but comparatively small, 

 broadly triangular in shape, with a few elevated straight parallel 

 ridges which diverge from the median line. Hinge-teeth numer- 

 ous, vertical in the middle, oblique towards the sides, lamellar, 

 straight or curved or rarely knie-shaped. Muscular impressions 

 distinct, the posterior one bounded within by an elevated ridge 

 which becomes weaker towards the beak. 



The form of the shell is very variable, some being more 

 rounded than others, but invariably more or less ovate and oblique 

 when compared with most of the species of this genus. The 

 majority of the specimens have the length slightly greater than 

 the height, but there are some in which the} 7 are nearly equal, 

 while in a few instances, the height is greater than the length. 

 The specimen shown in fig. 7 is probably an abnormal form of the 

 same species markedly ovate and more convex than others. 



The largest specimen which is a right valve measures 30 

 millim. in length and height and 9 millim. in depth. Another 

 specimen which is also a right valve measures 29,5 millim. in 

 length, 28 millim. in height and 7,5 millim. in depth, 



On the outer surface of some specimens we observe purplish 

 traces of an original colouring. This species is not unlike Pectun- 

 cidus mundus Sowerp.y (Ann. Mag. Nat. Hist., vol. XII. 7th. 

 series, 1903, p. 501) from Hirado in Western Japan in being 

 obliquely ovate, but it is larger and lacks the radiating ribs of the 

 latter. 



Very frequent, especially at Koshiba where, however, the 

 specimens occur in a much worn condition. 



Fossil occurrence. — Miyata Zone (Harashita in Shimo-Miya- 

 ta); Kanazawa Zone (Nojima); Koshiba Zone (Koshiba). 



Living. — Central Japan (Sobo Peninsula). 



