Chlamys daishakaensis Masuda & Sawada, 1961
MASUDA,
K. & Y. SAWADA. 1961. Some new Tertiary pectinids from Southwestern
Hokkaido, Japan. Japanese Journal of Geology and Geography, 32 (1):
19-29, pl. 4. [p. 23, pl. 4, figs. 8, 9]
Chlamys daishakaensis MASUDA and SAWADA, n. sp.
Pl IV, figs. 8, 9
Description :— Shell rather thick, higher than long, moderately inflated, subequilateral except for auricles, somewhat inequivalve, left valve a little more inflated than right; valves radiately ribbed and forming an angle of about 80° at apex. Right valve with about 24, prominent, distinctly scaled radial ribs, finely scaled intercalary threads and fine concentric growth lines; radial ribs narrower than their interspaces, usually accompanied with a finely scaled, faint, very fine radial thread on both sides of radial ribs near ventral margin; interspaces between radial ribs rather deep, with a single or a few, finely scaled, fine intercalary threads near ventral margin, in which primary ones usually appear at upper part of disc; posterior submargin sculptured with numerous, finely scaled, very fine radial threads; scales on radial ribs usually erected and somewhat inclined apically; auricles moderate in size; anterior auricle usually narrow in breadth, much larger and longer than posterior, furnished with narrow and deep byssal notch, very wide byssal area, sculptured with several, distinctly imbricated radial threads and concentric lines; posterior auricle truncated behind at about right to obtuse angle, sculptured with numerous, finely imbricated, fine radial threads, finely imbricated, fine intercalary threads and concentric lines; hinge with distinct but simple cardinal crura, wide and rather shallow resilial pit posteriorly sometimes contorted and with short, widely opening, fine but distinct lateral ridges and conspicuous ctenolium. Left valve nearly similar to right in sculpture except for radial ribs which are unequal in strength; anterior auricle much larger than posterior; both auricles sculptured with several, distinctly imbricated radial threads, finely imbricated, fine intercalary threads on upper part but with several, finely scaled, fine radial threads and fine concentric lines on lower. Interior surface of both valves gently folded.
Dimensions (in mm.) :-- Comparison and Affinity:— The present species is so closely related to Chlamys iwakina (YOKOYAMA) (1925, p. 19, pl. 3, fig. 13 that it is frequently misjudged with the latter. However, it differs from iwakiana by its right valve having nearly equal convexity with the left, the straight radial ribs
which are rather more elevated and more prominently scaled, the less prominent intercalary threads compared with iwkiana. Chlamys nipponensis KURODA (7932, p. 91) is another closely related species but it can be distinguished by its right valve which is less convex than the left, larger number of radial ribs which are somewhat less elevated, less prominently scaled and usually tend to bend towards antero-ventral margin, rather broad width of the anterior auricle in the right valve and the left valve having more unequal radial ribs, more prominent intercalary threads and the auricles with numerous, finely scaled, fine radial threads. Remarks:— The present species was described by S. NOMURA and. K. HATAI (1935) as Pecten (Pecten) iwakianus from the Daishaka formation, Aomori prefecture. However, by the. re-examination of the type specimens and the topotype specimens the writers consider that the so-called iwakianus of S. NOMURA and K. HATAI differs from M. YOKOYAMA'S species. This species is characterized by its subequivalved shell, about 24, elevated, prominently imbricated radial ribs which are usually accompanied with fine radial threads on both sides of radial ribs and narrower than their interspaces, narrow anterior auricle which is much larger and longer than posterior and by the left valve having the sculpture nearly similar to that of the right valve except for radial ribs which are rather unequal in strength. Type locality, Geological formation and Age:— Right stream cliff, about 2.3 km. north of Daishaka station along the Ōu Line, Namioka-machi, Minami- Tsugaru-gun, Aomori Prefecture (Lat. 40°46'26" N., Long. 140°35'15" E.). Daishaka formation. Early Pliocene. Distribution:— Daishaka, Narusawa and Hamada formations, Aomori Prefecture; Sawane, Haizume, Shiraiwa and Nishiyama formations, Niigata Prefecture; Sasaoka, Manganji and Abedate formations, Akita Prefecture; Setana formation, Hokkaido; Onma formation, Ishikawa Prefecture; Suenomatsuyama formation, Iwate Prefecture; Shigarami formation, Nagano Prefecture: Late Miocene to Early Pliocene. Occurrence:— Abundant in the granule conglomerate to conglomeratic very coarse-grained sandstone of the Daishaka formation; abundant in the very coarse-grained sandstone of the Sawane formation; abundant in the granule conglomerate to conglomeratic very coarse-grained sandstone of the Setana formation; few in the fine-grained sandstone of the Hamada formation; few in the conglomeratic coarse-grained sandstone of the Suenomatsuyama formation; few in the fine-grained sandstone of the Shigarami formation. KÔICHIRÔ MASUDA & YOSHIO SAWADA, 1961
|
K. Masuda & Y. Sawada, 1961,
plate 4. |