Placopecten nomurai Masuda 1953
MASUDA, K. 1953. On the Miocene Pectinidae from the Environs of Sendai; Part 3. Two Miocene Pectens. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 12: 83-87, pI. 8. [p. 83, pl. 8, figs. 1-7]
1953 Placopecten nomurai Masuda, 1953
K. Masuda, 1953, plate 8.
|
Shell rather small, thin, compressed, orbicular, subequilateral; the valves somewhat smooth, but radiately ribbed; pointed at top, forming an angle of about 100°. Right valve with a great number (about 50 or more) of fine, faint, close-set, flat-topped, smooth radial threads and usually with intercalary threads, both crossed by concentric fine growth lines; the radials are wider than the interspaces and usually tend to bifurcate at about the upper half of disc length; intercalary threads usually appear at about same time as the bifurcation of radials and rarely split into two riblets near margin; intercalaries
nearly equal to main radial threads in strength at margin. Left valve more convex than right, provided with numerous, faint, line, close-set, flattopped, smooth radial threads, and also rarely wlth intercalary threads; radials wider than interspaces and usually split into two riblets at about half of disc length. Auricles of right valve subequal in size, though the anterior is a little larger than the posterior; anterior furnished with a distinct byssal notch and narrow byssal area, and ornamented with distinct radial threads which rarely bifurcate and of concentric lines; posterior with distinct radial threads and concentric lines which make auricles appear imbricated. Anterior auricle in left valve with wide and shallow byssal notch which is a little larger than the posterior and sculptured with distinct, somewhat imbricated radials, being similar in sculpture with the posterior. Hinge of right valve with more or less strong ctenolium and distinct cardinal crura and deep resilial pit provided with distinct lateral ridges. Left valve with hinge provided with socket which correspond to lateral ridges of resilial pit of right valve. Internal surface smooth, except for marginal serration. Dimensions (in mm).--
*Holotype specimen
Type locality and geological horizon.— Stream floor of small tributary of the Natori River at Moniwa, Oide-mura, Natori-gun, Miyagi Prefecture (lat. 38°13'N., long. 140°47'E.). Moniwa formation. Early Miocene.
Depository.— Department of Geology, College of Education, Tôhoku University, Sendai, Japan. Reg. No. 1041. Remarks.— This species is characterized by its orbicular shape and thin shell which is provided with numerous, fine, close-set, flat-topped, dichotomous radial threads and intercalary threads, and by the strong cardinal crura. The left valve is characterized by rarely having intercalary threads, by the radials being wider than their interspaces and also by the thin shell. Placopecten clintonius SAY, according to W. C. MANSFIELD (1936, pp. 186-187, pl. 22, fig. 4) is without a defined type locality, but its true type locality may be "Old Kings Mill Wharf on the north shore of the James River. Virginia". Whatever be the case, clintonius SAY resembles nomurai in its general features, but can be distinguished by its smaller shell, less obtuse byssal notch and less numerous radial threads. Another similar species is Placopecten setanaensis KUBOTA (1950, pp. 183-185, pl. 7, figs. 1-4), from the Pliocene (?) Setana formation of southwestern Hokkaido, but nomurai can be distinguished therefrom by the larger shell, non-dichotomizing radials and by the obtuse byssal notch. Placopecten nomurai was referred by S. NOMURA (1940, p. 17) to his Pecten (Pecten) protomollitus, which he first described from the Miocene Tanosawa formation in western Aomori Prefecture and subsequently reported it from the Miocene Moniwa formation in the southern border of Sendai City, Miyagi Prefecture. The original description of S. NOMURA's protomollitus is as follows.-- "Shell small, about 49 mm. in height, orbicular, strongly compressed nearly equilateral; surface with fine, close-set, somewhat imbricated radial threads, some of which are slightly larger than the others in regular intervals. The smaller threads are generally three in number in the interspace between the larger ones. Interior smooth. Height, 39 mm." According to S. NOMURA, the shell is characterized by its orbicular outline, which is subtransparent and provided with fine and regular disposed radial threads. The characteristics of Placopecten protomollitus diverge from the specimens from the type locality of the Moniwa formation, especially in the type of radial threads between the interspaces, thus a new name has been given to them. Protomollitus from the Miocene Tanosawa formation in Aomori Prefecture, the holotype of the species, can be distinguished from the Moniwa specimens by having unequal radials, some of which are slightly larger than the others in regular intervals and the smaller threads are generally three in number in the interspaces between the larger ones.» KÔICHIRÔ MASUDA, 1953
|
«Remarks: – The present species is related to Placopecten setanaensis (Kubota), but setanaensis differs by its larger, somewhat posteriorly contorted shell, rather more distinct radial ribs and no intercalary threads. Placopecten protomollitus (Nomura) is also distinguishable from the present one by the right valve which is somewhat more convex than the left, fewer radial ribs which are little more distinct and sometimes bifurcate and by the left valve having finely imbricated radial ribs and intercalary threads.
Type locality, Geological formation and Age: – Stream floor of a small tributary of the Natori River at Moniwa, Sendai City, Miyagi Prefecture (Lat. 38°13'N., Long. 1400 47'E.). Moniwa member of Hatatate formation. Early Miocene. Distribution: – Moniwa member of Hatatate formation, Miyagi Prefecture; Shiratori member of Kadonosawa formation, Iwate Prefecture; Sugota formation, Akita Prefecture: all Early Miocene in age. Occurrence: – Abundant in the conglomeratic very coarse-grained sandstone of the Moniwa member; rare in the very coarse-grained sandstone of the Sugota formation; rare in the medium-grained sandstone of the Shiratori member.» MASUDA, K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku University [2nd. Series -Geology], 33 (2): 117-238, pls. 18-27. [p. 193]
|
Placopecten (s. s.) nomurai Masuda; K. Masuda, 1962, Tertiary Pectinidae of Japan, plate 24, figure 5.
|
«COMPARISON WITH FOSSIL SPECIES:— Present species, collected from the Moniwa Formation, was first described as Pecten (Pecten) protomolitus by Nomura (1940).
Placopecten setanaensis resembles the present species, but the latter can be distinguished therefrom by larger apical angle and absence of intercalary threads on both valves. Placopecten clintonius Say (Mansfield, 1936, p. 186-187, pI. 22, fig. 4) from the Late Miocene Yorktown Formation of Maryland, North America resembles nomurai in its general feature, but can be distinguished by its thicker shell, obtuse byssal notch, smaller number of nonbifurcated radial threads. Placopecten protomollitus (Nomura, 1940), Placopecten osawanoensis (Tsuda, 1959) and Placopecten sp. (Itoigawa, 1974) are not included in the Genus Placopecten but may belong to one new genus. Because left valve of P. protomollitus and P. osawanosensis are somewhat flatter than or equal to right one. Placopecten todaniensis Itoigawa and Nishikawa (1976) and Okamoto et at. (1983) really belongs to Chlamys. Only mode of multiplication of threads on left valve of P. protomollitus resembles that of Nipponopecten (pI. 31, figs. 3a and 7). In Japan as stated in earlier lines, only two fossil species, setanaensis and nomurai are considered to belong to Placopecten (pI. 31). P. nomurai resembles P. setanaensis and its variation of measured characters is harmonious with that of setanaensis except for apical angle (Text-figs. 46, 47 and 48). Left valve of setanaensis is also more convex than right one (Text-fig. 46).» SATO, Y. 1991. Paleontological study of molluscan assemblages of the Miocene Moniwa Formation, Northeast Japan and description of their Pectinidae. Report, Geological Survey of Japan, 272: 1-249, pls. 1-34. [p. 88, 91]
|
Placopecten momurai Masuda; Y. Sato, 1991, Paleontological study of molluscan assemblages of the Miocene Moniwa Formation, plate 27, figures 1-9; plate 28, figures 1-10.
|