Cryptopecten vesiculosus makiyamai Hayami, 1984
HAYAMI, I. 1984. Natural history and evolution of Cryptopecten (A Cenozoic-Recent Pectinid Genus). The University Museum, The University of Tokyo, Bulletin, 24: 1-149, pls. 1-13. [p. 109, pl. 7, figs. 11, 12]
1984 Cryptopecten vesiculosus makiyamai Hayami, 1984
I. Hayami, 1984, plate 7.
|
«Type.— The holotype is a left valve (UMUT CM 16033a) from the Hosoya Silt of the Kakegawa Group at the west of Ugari, northeast of Yamanashi, Fukuroi City, Shizuoka Prefecture, central Honshu (34°47.5'N, 137°56.3'E). 44.5 mm long, 43.1 mm high, 9.9 mm thick.
Material (Paratypes).— In addition to the holotype 25 specimens of the samples Kg 1 from the type locality and Kg 2 from a nearby locality are concerned with the following description and discussions. See also the list of examined samples (p. 127). Diagnosis.— Unusually large-sized subspecies of Cryptopecten vesiculosus characterized by rapid growth rate, relatively large number of radial ribs, thin test and narrow central solid ridge of radial rib. This subspecies characteristically occurs in silty sediments. Discription.— Shell often exceeding 40 mm in length and height; test comparatively thin; apical angle approximately 95 degrees; 15-18 radial ribs, with an average of 16, each of which consists of a comparatively narrow central ridge and a pair of relatively wide hollow parts; rib interspace marked with a number of fine radial threads in adult shell, showing lattice ornament with fine erect scales; other characteristics essentially similar to those of Phenotype Q of C. vesiculosus vesiculosus. Remarks.— Since Makiyama's (1931) stratigraphic work, it has been known that C. vesiculosus occurs frequently in the Hosoya Silt of the Kakegawa Group in Ugari area of Fukuroi City. No paleontological description, however, has been made on the material of this member. Owing to the small sample size and appreciable secondary deformation, detailed morphometric comparison with other samples is still difficult. The specimens from the Hosoya Silt (samples Kg 1 and Kg 2) are, however, morphologically and paleoecologically very unique in the following points. Firstly, the maximum shell size and growth rate are unusually large as shown in Table 13 and Fig. 7. In spite of the large size, the test is comparatively thin. Secondly, the central solid ridge of each rib is much narrower, and the hollow parts, though the covering imbricated scales have been lost in every specimen, are somewhat wider than in other samples of C. vesiculosus. Thirdly, the average number of radial ribs on the disk is slightly but significantly larger than that in any other sample of this species, as indicated in Table 11. Fourthly, as discusscd already (p. 80), the habitat and associated molluscs seem to be considerably different from those of Recent and other fossil populations of C. vesiculosus. This subspecies is found exclusively in silty beds. Because the age of the Hosoya Silt is certainly within the range of C. vesiculosus, I provesionally regard this phenon as constituting a geographical (or ecological) subspecies. Distribution.— This subspecies is known only from the Hosoya Silt Member of the Kakegawa Group in central Honshu, Japan. Late Pliocene (1.9-2.4 Ma).» ITARU HAYAMI, 1984
|