Pecten (Chlamys) lawsoni 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. 117, pl. 45, figs. 3, 4]
PECTEN (CHLAMYS) LAWSONI n. sp.
Pl. XLV, figs. 3 and 4. «Description.— Adult shell averaging about 70 millimeters in altitude, not as long as high, inequivalve, only slightly convex, equilateral, ears unequal; base regularly rounded; sides only slightly concave above; margins somewhat serrate. Right valve with about 18 to 20 subequal, prominent, narrow, round-topped, rugose ridges, separated by somewhat wider interspaces, in each of which runs a single more or less rugose riblet; some of the ribs are oceasionally medially sulcated or dichotomous; surface sculptured by numerous irregular incremental lines, which give the ribs a squamose appearance when they are unworn~ ears unequal, the anterior with three or four rather prominent rugose radials and numerous incremental lines; the posterior with several less prominent radials, and incremen~al sculpture; byssal notch rather profound. Left valve with 9 or 10 prominent narrow, round-topped, rugose major ribs, between each two of which there is a single minor rib similar to and almost equaling in prominence the major ribs; a single thread-like intercalary is visible in most of the interspaces; surface sculptured as in the right valve; ears similar to those of right valve except that there is no prominent notch between the anterior one and the disk.
Dimensions.— Alt. 70 mm.; long. 81 mm.; diameter 24 mm.; umborial angle 87°. This species is closely allied to P. hastatus, but is distinguishable by the paucity of its secondary sculpture (such as spines and secondary intercalaries), the nearly equal prominence of the major and minor ribs in the left valve, and the equality of the interspaces in the right. (The ribs in the right valve of P. hastatus occur in pairs, each pair being separated from those adjacent by spaces perceptibly wider than the interspaces between the members of each pair.) The smooth variety of P. hastatus mentioned as occurring in the Pliocene at San Diego, San Gregorio, etc., appears to occupy a position about midway between the typical P. hastatus and P. lawsoni. The type of P. lawsoni, which is now in the collection of the California Academy of Sciences, was collected by Mr. F. M. Anderson from the Pliocene at the Waldorf asphalt mine, 4 miles south of Guadaloupe, Santa Barbara County. Mr. Anderson has kindly furnished the following list of fossils from the beds at this mine: Crepidula princeps Conrad, Turritella cooperi Carpenter, Bittium asperum Gabb, Priene oregonensis Redfield, Trochita costellata Conrad, Lunatia lewisii Gould, Natica sp., Calliostoma gemmulatum Carpenter, Scala sp., Nassa perpinguis Hinds, Pleurotoma near carpenteriana Gabb, Pleurotoma near tryoniana Gabb, Pyrula (?) sp., Drillia torosa Carpenter, Purpura crispata Chemnitz, Columbella richthofeni Gabb, Dentalium sp., Acmaea sp., Panopea generosa Gould, Leda taphria Dall, Arca 2 sp., Venericardia ventricosa Gould, Pandora scapha Gabb, Tellina sp., Pecten 2 sp., Martesia ovoides Gould, Phacoides (+ Lucina) near acutilineatus Conrad, Phacoides sp., Macoma sp., Mytilus sp., Ostrea sp., Echinarachnius sp., and coral. The writer is also indebted to Mr. Anderson for the following list of fossils from a locality 1 mlle north of Schumann switch, (3 miles east of the Waldorf asphalt mine), Santa Barbara County: Trochita near costellata Conrad, Trochita near inornata Gabb, Calliostoma sp., Lunatia lewisii Gould, Glycymeris (+ Pectunculus) near patulus Conrad, Glycymeris sp., Venericardia ventricosa Gould, Pecten lawsoni n. sp., Pecten ashleyi n. sp., etc. Named in honor of Dr. A. C. Lawson, professor of geology at the University of California. RANGE.
Pliocene (lower). Waldorf asphalt mine, 4 miles south of Guadaloupe, and 1 mile north of Schumann switch, Santa Barbara County (F. M. Anderson).»
RALPH ARNOLD, 1906
|
R. Arnold, 1906, plate 45.
|