Leptopecten andersoni andersoni (Arnold, 1906)
ARNOLD, R. 1906. The Tertiary and Quaternary pectens of California. United States Geological Survey Professional Paper, 47: 1-264, pls. 1-53. [p. 82, pl. 26, figs. 5, 5a, 6, 7, 8, 8a]
1906 Pecten (Plagioctenium) andersoni Arnold, 1906
R. Arnold, 1906, plate 26.
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«Description.— Shell averaging about 85 millimeters in altitude, generally somewhat longer than high, both valves convex, more or less inequivalve and inequilateral; margins smooth; base regularly rounded; sides concave above. Right valve with 14 to 18 prominent rounded ribs, separated by flat-bottomed interspaces, which are generally of about equal width to, but sometimes narrower than, the ribs; surface sculptured by numerous, sharp, imbricating, incremental lines, and sometimes, at irregular intervals, by more or less prominent lines of interrupted growth; hinge line more than one-half length of disk; ears subequal; anterior ear with 3 or 4 sharp radials and numerous fine concentric lines; byssal notch quite pronounced; posterior ear sculptured similarly to anterior, and truncated nearly rectangularly. Left valve more convex than the right, sculptured similarly and with similar ears, except that the notch in the anterior is not so pronounced.
Dimensions.— Alt. 37 mm.; long. 44 mm.; hinge line 23 mm.; diameter about 8 mm.; umbonal angle 105º. A prominent characteristic of this species is that the left is decidedly the more convex of the two valves. In this respect it is similar to P. latiauritus and varieties. In fact, the young of P. andersoni is almost indistinguishable, in some instances, from small specimens of P. latiauritus var. monotimeris. The young of P. andersoni are equilateral, are relatively more convex, and have more prominent ribs than the adult forms. Those specimens of P. andersoni found in the earlier formations appear to be somewhat smaller, and have on an average fewer ribs (14 to 17) than those in the later Miocene horizons, which have from 15 to 18. Exceptions to this rule, however, are not rare, one specimen, for example, from the same locality as the type (which has 16 ribs on the left valve) having 18 ribs. It would not be surprising to the writer to find, after an examination of a large series of P. andersoni and allied forms from the upper Miocene, that it is more or less closely related to P. discus. Large specimens of P. andersoni very closely resemble some specimens of P. discus but differ from the latter in the fewer number (14 to 18) of their ribs, P. discus having 18 to 21. P. andersoni ranges from the lower to the middle (and probably upper) Miocene. At locality No. 9 (L. S. J. U. Geol. Survey), on "Tuff Hill," 3½ miles southeast of Stanford University. P. andersoni is associated with the following species: Pecten branneri, Chione n. sp., Phacoides acutilineatus, Venus {cf.) pertenuis, Terbratella (?) n. sp., Glycymeris sp., and Balanus sp. (cf.) estrellanus. At locality No. 4, not far from but stratigraphically considerably above No. 9, we find P. andersoni with the the following: Agasoma barkerianum, Arca microdonta, Chione sp., Callista n. sp. A., Dosinia mathewsonii, Leda (cf.) taphria, Phacoides acutilineatus, Mactra californica, Natica ocoyana (?), Saxidomas gibbosus, Tapes truncata (?), Trochita costellata, Tellina (Angulus) sp., Tritonium sp., Tivela ineziana and Yoldia n. sp. (aff.) cooperi. On the Searsville road, 3½ miles west of Stanford University (locality 2), P. andersoni is associated with the following among other species: Agasoma gravida, Agasoma kernianum, Dosinia (aff.) ponderosa, and Pinna alamedensis. At Barker's ranch, Kern County, Mr. Watts found P. andersoni associated with Agasoma barkerianum and kernianum and other supposedly characteristic lower Miocene forms. P. andersoni occurs with P. crassicardo in the upper Miocene 3 miles south of Mount Diablo. In Avenal Canyon, Kern County, P. andersoni is associated with P. andersoni var. barkerianus, Turritella ocoyana, Venus sp., Ostrea sp., and a large flat Dosinia. In the Devils Den country, Kern County, P. andersoni is found with P. crassicardo in beds of probable lower Miocene (Vaqueros) age. The type of P. andersoni, which is now in the collection of the department of geology, Stanford University, is from locality 127, three-fourths mile below Zayante station on Zayante Creek, Santa Cruz County, where it was found associated with P. sespeensis var. hydei and Balanus (cf.) estrellanus. The beds from which the type was taken lie only a few feet below, and conformable with, the base of the characteristic Monterey shales, which are very well developed in the vicinity. RANGE
Miocene (upper). Three miles south of Mount Diablo, Contra Costa County (University of California); Pacheco, Contra Costa County (J. P. Smith); San Pablo Bay, near Pinole, Contra Costa County (Merriam, Arnold).
Miocene (middle and lower). Locality 127, Zayante Creek, three-fourths mile below Zayante station; near Glenwood (Miss Hecox); near Magnetic Springs; near Felton (locality 32), Santa Cruz County; near Stanford University at localities 1, 2, 3, 4, 9, 34, and 35, Santa Clara and San Mateo counties (Branner, J. P. Smith, Arnold, and others); Barker ranch, Kern County (Watts); New Almaden (Turner); Contra Costa County (Lawson); near head of Slacks Canyon, NE. ¼ sec. 9, T. 22 S., R. 13 E., Mount Diablo meridian, Monterey County (Hamlin); Avenal Canyon, Kern County (Eldridge); Devils Den, Kern County (Eldridge).» RALPH ARNOLD, 1906
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