Parvamussium mideocenicum (Vokes, 1939)
VOKES, H. E. 1939. Molluscan faunas of the Domengine and Arroyo Hondo Formations of the California Eocene. Annals of the New York Academy of Sciences, 38: 1-246, pls. 1-22. [p. 55, pl. 3, figs. 2-4]
1939 Propeamussium mideocenicum Vokes, 1939
H. E. Vokes, 1939, plate 3.
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«Pecten cf. interradiatus Gabb: Dickerson (1916) 426, 431 (loc. 1817).
Pecten vacavillensis "Palmer"; Merriam & Turner (1937) 94 (Lillis Ranch). Syntype 15584: loc. 1817; length (incomplete) 9.5 mm.. altitude 8.8 mm. Syntype 15585; loc. 1817; length 8.1 mm.. altitude 6.2 mm. Syntype 15586; loc. 1817: length 7.5 mm., altitude 8.0 mm. DESCRIPTION.— Shell small, thin, slightly higher than long; ventral margin evenly rounded, the anterior and posterior margins straight to the umbos; surface of left valve ornamented with 10 to 12 low, rounded ribs separated by flat-bottomed interspaces of varying width, widest on the center of the valve but never less than twice the width of the ribs; both ribs and interspaces marked by faint lines of growth; surface of right valve not seen; ears smooth, subequal, the byssal notch well-defined, small; interior of both valves with 10 to 12 ribs extending from the umbo to the ventral margin and directly opposite the ribbing on the exterior of the valve.
COMPARISON.— Propeamussium mideocenicum may be distinguished by the fact that the ribbing on the surface of the left valve is directly opposite that on the interior of the valve, which extends from the umbo to the ventral margin. P. interradiatum (Gabb*) has but 8 internal ribs which extend only half way down the valve from the umbo. P. vacavillense (Palmer†) also has but 8 internal ribs which occupy only the ventral two thirds of the valve. P. lajollaense (M. A. Hanna‡) from the La Jolla formation, has about 65 ribs on the exterior of the valve.»
* Gabb (1869) 190. pl. 33, f. 98, 98a; as Peden interradiatus.
† Palmer (1923) 301. pl. 55. f. 3, 4a, 4b, 5; as Pecten vacavillensis. ‡ M. A. Hanna (1927) 277. PI. 32. f. 1, 3, 4, 5, 8; as Pecten lajollaensis. HAROLD ERNEST VOKES, 1939
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«Syntypes.— UCMP 15584 and 15586; not? 15585 which may be Parvamussium stanfordensis.
Type locality.— UC 1817. Fresno County, Calif. Cerros Shale Member, Lodo Formation (Keen and Bentson, 1944), Paleocene. "One of the early Eocene specimens of Propeamussium mideocenicum Vokes, 1939, UCMP 15585 from the Arroyo Hondo Shale Member of the Lodo Formation in the San Joaquin basin, California, might be P. stanfordensis. The ears, number and configuration of internal ribs, and size of this syntype are similar to P. stanfordensis. The other syntypes (both left valves) are quite distinct; they probably represent an entirely different species than P. stanfordensis judging by the occurrence of external ribbing and by the fact that these ribs are much more numerous than on the right valve—12 instead of 9." (Addicott, 1971, p. 229) Comparison.— "P. interradiatum (Gabb) has but 8 internal ribs which extend only half way down the valve from the umbo. * * * P. lajollaense (M. A. Hanna) from the La Jolla formation, has about 65 ribs on the exterior of the valve." (Vokes, 1939, p. 56) Parvamussium leohertleini, from the Eocene of Alaska, differs from P. mideocenicum principally in lacking a well-defined byssal notch. The left valve of P. mideocenicum can be differentiated from P. leohertleini by its prominent, sharp external ribs. Further, but minor, differences are the relatively shorter internal ribs and the prominent, but very fine, growth striae of P. leohertleini. (Addicott, 1971, p. 227) Comments.— The exterior of the left valve bears 12 preserved well-defined, small ribs but may have had originally about 15; the shell is glossy white. The left valve auricles seem to have been equal in size. Geographic range. — Middle California. Geologic range. — Paleocene. Occurrence in California.— Paleocene: Cerros Shale Member, Lodo Formation (Keen and Bentson, 1944).» MOORE, E. J. 1984. Tertiary Marine Pelecypods of California: Propeamussidae and Pectinidae. United States Geological Survey Professional Paper, 1228-B: iv + B1-B112, figs. 1-2, pls. 1-42. [p. B9]
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Parvamussium mideocenicum (Vokes); E. J. Moore, 1984, Tertiary Marine Pelecypods of California: Propeamussidae and Pectinidae, plate 1, figure 11.
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«One of the early Eocene specimens of Propeamussium mideocenicum Vokes, 1939, UCMP 15585 from the Arroyo Hondo Shale Member of the Lodo Formation in the San Joaquin basin, California, might be P. stanfordensis. The ears, number and configuration of internal ribs, and size of this syntype are similar to P. stanfordensis. The other syntypes (both left valves) are quite distinct; they probably represent an entirely different species than P. stanfordensis judging by the occurrence of external ribbing and by the fact that these ribs are much more numerous than on the right valve – 12 instead of 9.»
ADDICOTT, W. O. 1971. Some Paleogene mud pectens of the genus Propeamussium from Alaska and California. The Veliger, 13 (3): 226-230, figs. 1-13. [p. 229]
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