Patinopecten nakajimai Masuda, 1954
MASUDA, K. 1954. On the Miocene Pectinidae from the Environs of Sendai; part 6; a new pectinid from the Moniwa Formation. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 15: 159-162 [p. 158, pl. 21, figs. 1-5]
«Patinopecten nakajimai MASUDA, n. sp.
Plate 21, figures 1a-b, 2a-b, 3a-b, 4 & 5.
Shell large, rather thick, orbicular, equilateral except for auricles, a little higher than long; the right valve being more convex than the left; both valves radiately ribbed and forming an angle of about 100º at apex.
Right valve with about 40, low, roundtopped, close-set radial threads and fine concentric growth lines; the radials are wider than their interspaces in breadth at upper half of disc and nearly equal at the lower half, anrl usually tend to separate into two or three riblets at the upper half of the disc. The rib-bifurcation in the anterior and posterior areas are more conspicuous than in the central area of the valve. Left valve with about 40, distinct radial threads and fine concentric growth lines, and very rarely with faint intercalary threads between the radials; the interspaces are much wider than the radials and ornamented with fine network. Auricles of right valve subequal in size, though the anterior is a little larger than the posterior; anterior furnished with a wide and shallow byssal notch; ornamented with concentric lines and radial threads. Anterior auricle in left valve a little larger than posterior and sculptured with imbricated radials and concentric lines, the sculpture being similar to that of the posterior. Hinge of right valve with distinct cardinal crura, and wide and shallow resilial pit provided with lateral ridges and with no ctenolium. Left valve with hinge provided with sockets which correspond to lateral ridges of resilial pit of right valve and with cardinal crura. Internal surface of both valves smooth. Dimensions (in mm) :--
* Holotype specimen
Type locality and horizon:— River cliff of Natori-gawa near the Akiu Car-line Station at Kita-Akaishi, Oide-mura, Natori-gun, Miyagi Prefecture. Lat. 38º 13' N ., Long. 140º 45' E. Moniwa formation. Early Miocene.
Depository:— Reg. No. 1063; Department of Geology, College of Education, Tohoku University. Remarks:— This species is characterized by its rather thick, large, compressed shell which is provided with about 40 low, close-set, bi- or trifurcating radial threads in the right valve and by the left valve in having radial threads much narrower than the interspaces, and by its being less convex than the right. Patinopecten yamasakii (YOKOYAMA) resembles the present species in having bi- or trifurcate radial threads, but is distinguished therefrom by the less number of radial threads. which are low and faint. Placopecten akihoensis (MATSUMOTO) also resembles the present species in having an orbicular shell and very similar cardinal crura, faint radial threads, wide and shallow byssal notch in the right valve and by possessing about 40 undichotomized radial threads on the left valve. However, MATSUMOTO's species can be distinguished from the present one by the less convexity of the valves, much fainter raclial threads, radials narrower than the interspaces, lacking radial threads on the young species of the right valve, by the occasional intercalaries on the left valve, by the radials being about equal to or slightly narrower than their interspaces and by the regularity of the radials. This species is rather rare in the Moniwa formation and is known only from the type locality and Ainosawa, a small valley extending northwards about 500 m northeast of the Kita-Akaishi Car-line station in Oide-mura, Natori-gun, Miyagi Prefecture.» |
K. Masuda, 1954, plate 21.
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KÔICHIRÔ MASUDA, 1954