26 



The following figures show that the ancient specimens are not only larger, 

 but that the proportions of the shell are also different ;— the shell having increas- 

 ed in length, in proportion to its hight. 



Length. Hight. 



Average dimensions of ten ( Eecent, 61.8 50.4 



largest specimens, \ Mound, 65.7 52. 



, n , nnn ,. u ■ ( 30 Eecent. 81.2 



Assuming length to he 100, hight in, | 2g Mound _ 79 ± 



^. . . , , . ( Eecent, 6?. 5 54. 



Dimensions of largest specimen, } -tf_om\&, 80.5 69 



These differences, though slight, appear to be constant, and indicate a change 

 in the character of the shell, as to its relative size, number of ribs, size of hinge 

 area, and proportionate diameters. 



Area inflata Reeve. Plate xvm. Fig. 5. 



This species is comparatively rare in the Mounds though of common occur- 

 rence along the shores. It is a common edible mollusk in the markets of Tokio. 

 The average number of ribs in the recent specimens from Omori is 41.2 Eeeve 

 gives 40 as the number in his description of the species, though Schrenck states 

 that they average 42 to 43. 



In the few specimens found in the Mounds the ribs averaged 39.6 This 

 average, drawn from so few specimens, is of but little value, yet, taken in con- 

 nection with the other species of Area, wherein the ancient specimens exhibited 

 a less number of ribs, it is of some interest. 



In the dimensions of the hinge area, however, the most remarkable change is 

 seen between the ancient and recent forms. The average width in nine recent 

 specimens, measuring one valve, was 6.4, while in five ancient specimens it 

 was 15. These dimensions in the recent specimens ranged as follows : 8, 5, 4, 

 7, 6, 6, 5, 6, 11 ; and in the ancient specimens, 17, 16, 13, 17, 12. 



Area granosa Limit. 



This is one of the most abundant shells in the Mounds. I have never met 

 with it in my collections made at Yenoshima, Yedo Bay, or Yezo. Takamine 

 did not obtain it in his dredgings and shore collecting at Funagawa and Niigata, 

 on the west coast. Sasaki did not find it at Sendai Bay. Prof. Yatabe and 



