Parvamussium robinsonense Sundberg, 1989
SUNDBERG, F. A. 1989. Propeamussium Species (Bivalvia: Propeamussiidae) from the Upper Cretaceous of Southern California. Journal of Paleontology, 63 (1): 53-63, figs. 1-9. [p. 59, figs. 6, 7]
1989 Propeamussium (Parvamussium) robinsonense Sundberg, 1989
F. A. Sundberg, 1989, figure 6.
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«Diagnosis.— Propeamussium (Parvamussium) with subcircular valve outline; slight byssal notch in right anterior auricle in later growth stages; 0-2 riblets on right anterior auricle; 9-11 internal ribs on right valve (9-11 primary); 9-12 internal ribs on left valves (8-12 primary and 0-2 secondary); dorsal lateral margins of auricles projecting above umbo on right valve.
Description.— Valves subcircular in outline, approximately as wide as high (height/width = 1.13); auricles relatively large, equal; average umbonal angle 107°; average maximum internal rib height 66 percent of left valve height; secondary internal ribs present in some left valves.
Exterior of right valves covered with several fine submarginal lirae of moderate relief; ventral margins fragile beyond distal ends of internal ribs, ventral margins not preserved on specimens; posterior auricles with fine growth lines and acute to obtuse auricle angles; anterior auricles with fine growth lines, 0-2 radial costellae, slight byssal notches, acute to 90° auricle angles; dorsal lateral margin of auricles projecting above umbo; interior of right valve with 9-11 ribs, all primary; internal ribs straight; lateral internal ribs swollen slightly at termination.
Exterior of left valves covered with several fine radial costae; radial costae variable in height, typically with one or two weaker costae between two stronger costae; radial costae sharp in cross section; growth lines faint to raised, fine submarginal lirae; grid pattern formed with radial costae and submarginal lirae; posterior auricles with faint growth lines, slightly acute auricle an- gles; anterior auricles with faint growth lines, 90° to slightly obtuse auricle angles; interior of left valves with 9-12 ribs (8-12 primary and 0-2 secondary); internal ribs straight, with no terminal swelling. Measurements.— See Table 2.
Discussion.--Propeamussium cowperi (Waring, 1917) differs from P. (P.) robinsonense in having 4-8 internal ribs on the left valve, only nine internal ribs on the right valve, the internal ribs extending a maximum 80 percent of the left valve height, radial costellae on the posterior auricle of the right valve, and the lack of terminal swelling on the internal ribs. Propeamussium (Parvamussium) yubarense (Yabe and Nagao, 1928) differs from P. (P.) robinsonense in having faint radial costae on the right valve, a pronounced byssal notch in some specimens (Tamura, 1976, PI. 1, figs. 10, 11; probably a new species), fewer internal ribs (typically 8-9), unequal auricles, and different auricle angles (Yabe and Nagao, 1928; Tashiro, 1976). Propeamussium (Parvamussium?) awajense Ichikawa and Maeda (1958) differs from P. (P.) robinsonense in having coarse submarginal lirae and fine radial costae on the left valve (if Ichikawa and Maeda are correct in identifying the valves as left valves, no right valves are known) and a smaller umbonal angle (90°). Propeamussium (Parvamussium) kimurai (Hayami, 1965) differs from P. (P.) robinsonense in having unequal auricles, well-developed byssal notch, smaller umbonal angle (100?), fewer ribs (8-9), radial costellae on left valve auricles, and coarser radial costae on left valve. Propeamussium (Parvamussium?) kattoi (Tashiro and Matsuda, 1986) differs from P. (P.) robinsonense in having coarser concentric costae on right valve, 8-9 internal ribs, straight hinge line, small umbonal angle (85°), and coarser radial costae on left valve.
Etymology.— The species is named after its type locality at Robinson Ranch.
Material.— Holotype, LACMIP 7222, right valve from UCLA loc. 7074; paratypes, LACMIP 7209-7211 from CSUF loc. 68.
Age.— Late medial Santonian, Bostrychoceras elongatum Biozone.
Occurrence.— Holz Shale Member of the Ladd Formation at the Robinson Ranch, Plano Trabuco (UCLA loc. 7074); Silverado Canyon (CSUF loc. 68), Santa Ana Mountains, Orange County, California.»
FREDERICK A. SUNDBERG, 1989
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