Interchlamys mediacostata grewingki (Hertlein, 1966)
HERTLEIN, L. G. 1966. Pliocene fossils from Rancho El Refugio, Baja California, and Cerralvo Island, Mexico. Proceedings of the California Academy of Sciences [4th Series], 30 (14): 265-284, 17 text figs. [p. 276, fig. 12]
1966 Chlamys mediacostata grewingki Hertlein, 1966
L. G. Hertlein, 1966, figures 10-17.
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«DESCRIPTION. Shell, a right valve, higher than long, nearly equilateral, gently convex; with about 17 or 18 low, rounded, radial ribs widest at the middle of the ventral margin and separated by interspaces narrower than the ribs; surfaces of the ribs with traces of concentric imbricating lines of growth which loop upward; each interspace on the ventral half of the shell with a well developed riblet; the ears are imperfectly preserved but the direction of the lines of growth on the posterior ear indicate a nearly squarely truncated posterior margin, a well developed notch is present under the remnant of the anterior ear. No sculpture is visible on the ears; if originally present, it has been eroded. Dimensions: length 34.2 mm., height 39.6 mm., apical angle 81.5°.
HOLOTYPE. A right valve, no. 12812, California Academy of Sciences, Department of Geology Type Collection from Locality 39413 (CAS), along the trail about 200 meters to the southeast from Locality 39412 (CAS), southeast of the ranch house, Rancho El Refugio, Baja California; G D. Hanna and J. W. Durham, collectors; Pliocene. COMMENTS. The shell of this new subspecies bears a decided resemblance to Pecten mediacostatus Hanna (Proc. Calif. Acad. Sci., Fourth Ser., vol. 14, no. 18, p. 472, pl. 22, fig. 6; pl. 24, fig. 2, 1926; see also Durham, Geol. Soc. America, Mem. 43, pt. 2, p. 65, pl. 7, fig. 4, 1950), which was originally described from "Alverson Canon on the south side of Coyote Mountain, Imperial County, California, in the Pliocene coral reef about midway up the canyon." It differs from that species in the less numerous ribs (17 or 18 rather than 23 or 24) and in the narrower apical angle. All the specimens of C. mediacostata in the collections of the California Academy of Sciences, many rather imperfectly preserved, have more numerous ribs than the present specimen. Durham mentioned a specimen with 24 ribs from the San Marcos formation on San Marcos Island in the Gulf of California. A species described from strata of Pliocene age in Florida, Pecten (Lyropecten) tamiamiensis Mansfield (U.S. Geol. Survey, Prof. Paper 170-D, p. 47, pl. 16, figs. 4,6, 1932), 69 mm. long and 74 mm. high, with 23 ribs, is quite similar to C. mediacostata. In view of the fact that only a single valve is represented in the present collection and its decided similarity to C. medicostata, it seems best to describe it as a subspecies of that species. Similar species of late Tertiary age have been described from the Caribbean region. Pecten interlineatus Gabb (see Pilsbry, Proc. Acad. Nat. Sci. Philadelphia, vol. 73, pt. 2, p. 411, pl. 45, fig. 3, 1922), from strata of Miocene age in Santo Domingo, 23 mm. long and 24.7 mm. high, was described as possessing 15 to 16 radial ribs and the posterior ear is said to be sculptured with coarse riblets. A subspecies, Pecten interlineatus aidei (Williston MS.) Harris (Bull. Amer. Paleo., vol. 13, no. 49, p. 29, pl. 15, figs. 6, 9, 1927) from beds of Miocene age in Venezuela is said to be similar to but about twice as large as the type specimen of P. interlineatus. A geologically earlier member of this group of pectens is Pecten crocus Cooke (Carnegie Inst. Washington, Publication no. 291, p. 135, pl. 9, figs. 2a, 2b; pl. 11, fig. 9, 1919), 36 mm. long and 39 mm. high, from the island of Anguilla in the West Indies which was described as possessing 22 ribs. The age was given as Oligocene. This subspecies is named for Constantin Grewingk, author of an early report containing observations on the geology of portions of the Gulf of California region.» LEO GEORGE HERTLEIN, 1966
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