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Contributions in Science, Number 520 Squires: Pico Formation Paleontology & 81

(fide Waller, 1995); Holocene, southern California and Gulf of specimens of each ‘‘species’’ that shows they lack consistent,
California to Peru (Coan et al., 2000:235). reliable morphologic differences separating them from one
another. Their reported differences (see Hertlein, 1925; Hertlein
Argopecten invalidus (Hanna, 1924) and Grant, 1972) were based on whether or not the ribs are flat-
Figures 16–19 topped, rounded, and on the depth of the interspaces. These
differences, however, are attributable to how much weathering
Pecten (Plagioctenium) cooperi Arnold, 1906:124, pl. 49, figs. 2–
4. Not Pecten cooperi Smith, 1903. the specimens have experienced. In the study area, for example,
Pecten invalidus Hanna, 1924:177, new name for P. cooperi specimens of A. invalidus at any one locality show variation in
Arnold, 1906. the shape and depth of the ribs, with the variation clearly
Pecten (Plagioctenium) subdolus Hertlein, 1925:20, pl. 5, figs. 2, attributable to the degree of weathering.
4, 7. In addition to A. invalidus, A. callidus, and A. subdolus, Grant
Pecten (Plagioctenium) callidus Hertlein, 1925:22, pl. 5, figs. 1, and Gale (1931:see pages 210, 211, 212, 214) reported three
3, 5, 6. other argopectinid species in the study area beds: A. percarus
Pecten (Plagioctenium) invalidus Hanna. Jordan and Hertlein, (Hertlein, 1925), A. mendenhalli (Arnold, 1906), and A. imposter
1926:441; Minch et al., 1976:table 15. (Hanna, 1924). Argopecten percarus differs from A. invalidus by
Pecten (Aequipecten) purpuratus Lamarck variety subdolus having 24–25 ribs on the left valve and an umbonal angle of
Hertlein. Grant and Gale, 1931:211, pl. 5, fig. 1 (west of 118u. Argopecten mendenhalli differs from A. invalidus by
San Fernando Pass). having a much longer hinge line, weak sculpture on the right-
Pecten (Aequipecten) purpuratus Lamarck variety callidus valve anterior auricle, obsolete sculpture on the left-valve
Hertlein. Grant and Gale, 1931:211, pl. 5, fig. 4. anterior auricle. Argopecten imposter differs from A. invalidus
Pecten (Aequipecten) deserti Conrad variety invalidus Hanna. by having weak grooves along the sides of the major ribs and a
Grant and Gale, 1931:213–214, pl. 5, figs. 5a–c, 6a–c. left valve with narrower interspaces.
Aequipecten callidus (Hertlein). Wilson, 1955:tables 7, 8. Argopecten invalidus is similar to A. deserti (Conrad, 1855)
Aequipecten subdolus (Hertlein). Wilson, 1955:table 8. and the extant A. ventricosus (G.B. Sowerby II, 1842).
Argopecten invalidus (Hanna). Vedder, 1960: table 151.1; Argopecten invalidus differs from A. deserti by having larger
size, right-valve interspaces narrower than the ribs, left-valve ribs
Moore, 1984:B37, pl. 10, fig. 5; Squires et al., 2006:11–12, narrower than those on the right valve, left-valve interspaces
figs. 15, 16.
Pecten (Argopecten) subdolus Hertlein. Moore, 1968:50, pl. 23, wider than the ribs, more ribs on the right-valve anterior auricle,
figs. a, b. and a shorter hinge line. Argopecten invalidus differs from A.
Chlamys (Argopecten) callida Hertlein. Hertlein and Grant, ventricosus by having a larger maximum height (95 mm), less-
1972:198–199, pl. 32, figs. 9, 11. inflated right valve, as well as narrower and generally more ribs
Chlamys (Argopecten) invalida Hanna. Hertlein and Grant, on the right-valve anterior auricle.
1972:200–201, pl. 33, figs. 1, 3, 8. In the comparision of the above-mentioned argopectinids, only
Chlamys (Argopecten) subdola Hertlein. Hertlein and Grant, the ribs that extend continuously from the beak to the venter
1972:201–202, pl. 30, figs. 7, 8; pl. 35, figs. 2, 5, 9. were counted. Specimens with one or two weak, noncontinuous
Argopecten subdolus (Hertlein). Moore, 1984:B37–B38, pl. 10, ribs that are present on both the anteriormost and posteriormost
figs. 3, 4. sides of the specimens were not included. The largest specimen of
Argopecten callidus (Hertlein). Moore, 1984:B38–B39, pl. 10, A. invalidus in the study area is 70 mm in height.
TYPE MATERIAL. Holotype of Pecten (Plagioctenium)
figs. 7, 9.
cooperi Arnold, 1906: CAS 61855.01 [ex CAS/SU 8]; holotype
EMENDED DESCRIPTION. Shell medium size, up to height of Pecten (Plagioctenium) subdolus Hertlein, 1925: CAS
117 mm; specimens commonly approximately height 45–55 mm. 61881.01 [ex CAS/SU 51]; holotype of Pecten (Plagioctenium)
Valves slightly longer than high on most specimens; smaller callidus Hertlein, 1925: CAS 61882.01 [ex CAS/SU 53].
specimens tend to be slightly longer than high, larger specimens TYPE LOCALITY. Of Pecten (Plagioctenium) cooperi: Pacific
tend to be slightly higher than long. Left valve more convex than Beach, San Diego, San Diego County, California; San Diego
right valve on most specimens; valves nearly equally convex on Formation, Pliocene. Of Pecten (Plagioctenium) subdolus: CAS
few specimens. Hinge line approximately half of disk length. loc. 61881 [ex SU loc. 115], San Diego County, California; San
Umbonal (apical angle) 100u–105u. Ribs 20–22 in number on Diego Formation, Pliocene. Of Pecten (Plagioctenium) callidus,
both valves, with lamellae in interspaces. Ribs become obsolete CAS loc. 61882 [ex SU loc. 116], Cedros Island, Baja California,
on anteriormost and posteriormost parts of valves and tend to Mexico, Almejas Formation, Pliocene.
flatten out and become more convex in the later stages of GEOLOGIC AGE. Early to late Pliocene.
growth. Auricles with prominent radial riblets on both valves; STRATIGRAPHIC DISTRIBUTION. LOWER PLIOCENE:
riblets stronger on anterior auricles of both valves. Left valve: Almejas Formation, eastern Cedros Island and Tortugas Bay,
ribs narrower than on right valve; interspaces wider than ribs Baja California Sur, Mexico (Hertlein, 1925; Jordan and
and wider than those on right valve; anterior auricle with very Hertlein, 1926; Minch et al., 1976); Tirabuzon Formation
small notch; posterior auricle slightly truncated; anterior and [formerly Gloria Formation], Baja California Sur, Mexico
posterior auricles, both auricles with seven to nine riblets. Right (Wilson, 1955). UPPER PLIOCENE: Pico Formation, Holser
valve: ribs wider than on left valve; interspaces narrower than Canyon area, Los Angeles County, (Grant and Gale, 1931); Pico
ribs and narrower than those on left valve; anterior auricle with Formation, northern Simi Valley (especially Las Llajas Canyon),
small notch; five to seven flattish riblets, strongest one Ventura and Los Angeles counties, California (new information);
coincident with notch area and variable in width and elevation; and Valencia and Newhall areas, northern Los Angeles County,
posterior auricle slightly truncate; six to seven (rarely more) California (Grant and Gale, 1931; Squires et al., 2006; present
radial riblets. report); Niguel Formation, San Juan Capistrano, Orange County,
COMPARISON. Argopecten invalidus, A. callidus, and A. California (Vedder, 1960); San Diego Formation, lower member,
subdolus are conspecific based on a comparative study of actual San Diego County, California (Hertlein and Grant, 1972;