Azumapecten foeda (Yokoyama, 1926)
YOKOYAMA, M. 1926. Fossil
shells from Sado. Journal of the Faculty of Science, Imperial University of
Tokyo [Section II: Geology, Mineralogy, Geography, Seismology], 1 (8): 249-312, pls. 32-37. [p. 305, pl. 37, fig. 1]
1926 Pecten foedus Yokoyama, 1926
1958 Chlamys (Mirapecten) robustus Ozaki, 1958
1958 Chlamys (Mirapecten) robustus Ozaki, 1958
M. Yokoyama, 1926, plate 37.
|
«Only a single left valve which, however; is characteristic enough to be created into a new species.
The shell is moderately large, thick, fairly convex, orbicular, slightly higher than long, nearly equilateral, ornamented with thirteen strong, straight, elevated, rounded, unequally coarse-scaly, radiating ribs separated by deep valleys of greater breadth. Incremental lines fine, dense, somewhat wavy and most distinct in valleys. Ears large, unequal, the anterior larger, with a few coarse-scaly, strong, radiating riblets. Inner surface with a large oval muscular impression and traces of external ribs. Ventral border coarsely crenate, caused by tho ribs. Height 93. Length 87 millim. Depth 20 millim. Fossil occurrence.— Kaidaté.» MATAJIRO YOKOYAMA, 1926
|
«Discussion.— The present species has been referred to the subgenus Chlamys [Röding, 1798] (e.g., Masuda, 1962; Matsuura, 1985; Amano et al., 1991). However, it is transferred here to the subgenus Leochlamys MacNeil, 1967 (type species: Chlamys (Leochlamys) tugidakensis MacNeil, 1967 by original designation), based on the large anterior auricle with a broad byssal fasciole, a deep byssal notch and strong ctenolium in the right valve, highly elevated, regular radial costae with strong imbrications or spines, and lacking crenulations in the inner ventral margin, and shagreen microsculpture at least in the middle and late ontogenetic stages.
Chlamys (Leochlamys) foeda closely resembles Chlamys chiyodai Masuda and Sato, 1977 from the Upper Miocene Ôtsutsumi Formation in northeastern Japan. Masuda and Sato (1977) stated that C. chiyodai is distinguished from C. (L.) foeda by having “its few, fine riblets on the radial ribs of the right valve, much less prominent and less number of scales on the radial ribs and less lamellated concentric growth lines.” C. (L.) foeda from the Lower Pleistocene formations has, indeed, strong scaly sculpture on the radial costae. However, the specimens from the Lower Pliocene Shigarami Formation have weakly imbricated radial costae as in C. chiyodai (Amano et al., 1991). Unfortunately, C. chiyodai was proposed based only upon a single articulated specimen (holotype: SHM 21950; removed to NSMT), and its morphological variation is not known. Thus, further reexamination based on additional specimens is needed to determine the relationship between the two species. Chlamys (Leochlamys) tugidakensis MacNeil, 1967, the type species of the subgenus Leochlamys MacNeil, 1967, from unnamed Pliocene deposits in Alaska, is distinguished from the present species in having much more irregular radial costae. The large auricles with a deep byssal notch, strong ctenolium, and a broad hinge plate with an oblique resilifer of Chlamys (Leochlamys) foeda are shared by Chlamys arakawai (Nomura, 1935), from the uppermost Lower–lowest Middle Miocene Tanosawa Formation in the western Tsugaru area, northeastern Japan. However, C. (L.) foeda is distinguished from C. arakawai by having less numerous, higher, non-bifurcated radial costae. Occurrence.— Pliocene: Shigarami Formation; Naarai Formation; Tomioka Formation; Yunokogawa Formation; Early Pleistocene: Tomikawa Formation; Haizume Formation; Zukawa Formation; Sawane Formation.» MATSUBARA, T. 2009. Neogene Mollusca from the Yunokogawa Formation in the Shimokita Peninsula, Northeast Japan. Paleontological Research, 13 (2): 173-192, figs. 1-9.
|
Chlamys (Leochlamys) foeda (Yokoyama, 1926); T. Matsubara, 2009, Neogene Mollusca from the Yunokogawa Formation in the Shimokita Peninsula, Northeast Japan, figures 5.7, 6.6.
|
«Remarks:
Ozaki (1958) described Chlamys (Mirapecten) robustus as a new species from the Na-arai Formation in Chiba Prefecture. According to his description, this "new species" differs Chlamys foeda by having a flatter shell, narrower apical angle and more numerous ribs (15 in robustus and 13 in foeda). However, some specimens of the foeda from the Shigarami. and the Hitachi Formations have 15 radial ribs and show nearly same apical angle to the robustus. Therefore, it is hard to distinguish the robustus from the foeda. Affinities:
The present species is closely related to Chlamys (Chlamys) niikappuensis (PI. 1, fig. 5) from the early Middle Miocene Noya Formation in Hokkaido described by Masuda (1962). The latter species has 12 strong and imbricated radial ribs. However, the interspaces of the latter are ornamented with 4 radial ribs, 5 weak radial threads and network sculpture which are never observed in the foeda. Chlamys chiyodai, described from the Middle Miocene Otsutsumi Formation in Miyagi Prefecture, is another allied species to the present one. According to Masuda and Sato (1977) who proposed the chiyodai as a new species, this species can be distinguished from the foeda by its few, fine riblets on the radial ribs in the right valve and less lamellated growth lines. Surely, the above two Miocene forms are discriminated from the present one. However, both description of these forms were based on only one specimen respectively. Therefore, it is too difficult to conclude whether the Miocene forms can be distinguished from the foeda as an independent species or should be included in an infraspecific rank. Except for the above Miocene forms, there is no related species to the present one in and around Japan. Chlamys (Leochlamys) tugidakensis MacNeil (1967) from the Pliocene in Alaska resembles the present species in having an elongate anterior auricle of right valve and imbricated radial ribs. However, the former has more numerous and shows narrower apical angle than the latter. Moreover, as pointed by MacNeil (1967), the Alaska species seems to be a descendant of Chlamys columbiana (Clark et Arnold) from the Early Miocene Sooke Formation of Vancouver Island. Distribution:
Early Pleistocene Sawane, Haizume and Tomikawa Formations; Pliocene Shigarami, Zukawa, Na-arai and Hitachi Formations.» AMANO, K., K. TANAKA & S. KARASAWA. 1991. Chlamys (Chlamys) foeda (Yokoyama) from the Shiragami Formation in Nagano Prefecture, Central Japan. Bulletin of Joetsu University of Education, 10 (2): 305-313, pl. 1. [p. 308, 309]
|
Chlamys (Chlamys) foeda (Yokoyama); K. Amano, K. Tanaka & S. Karasawa, 1991, Chlamys (Chlamys) foeda (Yokoyama) from the Shiragami Formation in Nagano Prefecture, Central Japan, plate 1, figures 1-4.
|