130 Art - 6.— M, Yokoyaina : 



231. Nucula mirabilis, Adams et Beeve. 

 Pl. XIX. Fig. 9. 



Nucula mirabilis. Adams and 'Reeve, Zool. Voy. Samarang, Mollusca, p. 75, pl. XXI, fig. 8. 

 Tokunaga, Foss. Env. Tokyo, p. 56. 



Nucula cobboldiae. Brauns, Geol. Envir. Tokio p. 46. (non Sowerby). 



This is a species which not only resembles the preceding but 

 also to the English Crag species Nucula cobboldiae Sow. (Wood, 

 Crag Moll., Bivalves, p. 82 pl. X, fig. 9), so that Brauns took it 

 for the same species. Compared with the latter, there is no dif- 

 ference at all in its surface sculpture of divaricating and zigzag 

 striae. In full grown specimens of the Japanese fossil, the line 

 from which the striae (or riblets) diverge is approximately in the 

 median line of the shell as in the English fossil. The zigzag 

 striae are close in front of the median line, near the middle portion 

 of the ventral and the posterior margin, and also sometimes in other 

 parts, especially near the ventral margin. The size attained by full 

 grown specimens is also nearly the same, the largest Japanese shell 

 being about 30 millim in largest diameter. The number of teeth 

 in the latter is about twenty-five in the anterior row and about 

 ten in the posterior, so that the anterior has somewhat more teeth 

 than the English species. But the main differences between the 

 two are the presence of a shallow depression running from the 

 beak to the ventral margin just behind the posterior margin and the 

 more oblique and elongated form of the ligamental pit in the Japa- 

 nese species. Moreover, the inner margin of the latter, though 

 smooth on a cursoiy view, is finely and irregularly crenulated, a 

 character which is not found in the English form. 



Compared with Nucula insignis, Nucula mirabilis grows larger, 

 with the area more sharply bounded and the inner margin very 

 finely crenulate. 



The fossil specimens in our possession are very few and bro- 

 ken. Therefore, taking measurements from a nearly perfect 

 specimen obtained from the Upper Musashino of Shinagawa, Ave got 

 the following dimensions: Largest diameter 30 millim., smallest 

 diameter 22 millim., thickness 17 millim. ; heighl or length of area 

 11,5 millim., breadth of the same 9 millim. 



