On the Variation of the Fossil Glycymeris yessoensis

Abstract

The material upon which this investigation is based consists of seven series of detached valves of Glycymeris yessoensis (SOWERBY) obtained from the Upper Pliocene and Pleistocene rocks of this country. Series A, B, and C were collected from three fossil localities in Oga Peninsula, North Japan, and the remaining four series from the Pleistocene sands at Itabasi near Tokyo, and three localities in Boso Peninsula. The length of the shell was measured as usual, but diameter B (bb′) was taken as the greatest dimension that can be found starting, with the apex of the beak to a point on the ventral margin, instead of measuring the height. The present species is easily distinguished from related species in that the height approximately coincides with diameter B. Fig. 2 (p. 111) shows the distribution of the length/diameter B ratio in the G-series. Means and standard deviations of the seven series were estimated, and then each compared with the other. Of the Pliocene of Oga, the difference between A and B is significant, but that of A and C appears to be slight. The fact is in harmony with the stratigraphical observation that A and C lie underneath a lignite seam while B occurs in an upper horizon. The difference between D and F is very insignificant, so that they may be taken as belonging to one and the same race. It was observed, however, that the differences between other series of the Pleistocene localities are more or less significant. The above fact agrees very well with other stratigraphical criteria. Tables 1, 2, and 3 show the correlation of length and number of ligament rides in G, C, and B-series. The difference between the correlation coefficients of G and B were proved to be very significant. Fluctuation of crenations of the ventral margin does not exhibit any difference. The number of teeth is very variable. The writer thinks that the conchometrical methods are useful in palaeontology and stratigraphy.