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Show 374 TERTIARY SPECIES? Genus MACTRA, Linnaeus. MACTBA GIBBSANA, Meek. Plate 2, figs. 8, 8 a, and 8 b. Mactra Gibbiana, Meek ( 1861), Proceed. Acad. Nat Sei. Philad., xiii, 315. Shell transversely oval, or subtrigonal, moderately convex, rather thin; anterior side narrowly rounded; base forming a regular semi- elliptic curve; posterior side slightly truncated at the immediate extremity, abruptly rounded or subangular at its connection with the base below; dorsal outline sloping from the beaks in front and behind at an angle, of about 120°; beaks central, rather elevated, bnt small, and not projecting much above the hinge- margin; surface marked only by moderately distinct lines of growth. Posterior muscular impression oval, well defined; pallial line distinct, and provided with a rather deep, horizontal sinus, which is about one- third longer than wide. Length, 2.04 inches; height, 1.50 inches; breadth or convexity, 1 inch. Some eight or ten more or less perfect valves of this species were found near Port Discovery on the Straits of Fuca in a loose mass of rather hard, fine, gray sandstone, in which ( when moistened and examined with a magnifier) numerous black grains may be seen. So for as I am able to determine, it belongs to an undescribed species, which I proposed to name in honor of Mr. George Gibbs, the geologist of the Boundary Survey. As the matrix in which it is embedded contains DO other fossils, excepting fragments of an unknown Univalve, I have been unable to decide whether it is of Tertiary or Cretaceous age, though I incline to the opinion that it belongs to the former. Mr. Conrad has described a similar Mactra under the name of M. albara from the Tertiary on the Columbia Biver ( Am. Jour. ScL, vol. v, 2d series, 432). The species before me, however, is proportionally longer and less gibbous. It also differs in being destitute of a distinct angle down the posterior umbonal slopes. It is likewise proportionally longer than any of the Cretaceous species described by Dr. Hayden and myself from Nebraska. Locality and position.- Found loose on the Straits of Fuca; Cretaceous or Tertiary. |