874 



TERTIARY SPECIES ? 



Genus MACTEA, Linnaeus. 

 Mactea Gibe SANA, Meek. 



Plate 2, figs. 8, 8 a, and 8 h. 

 Mactra Gibhsana, Meek (1861), Proceed. Acad. Nat. Sci. 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, but small, and not pro- 

 jecting much above the hinge-margin ; surface marked only by moder- 

 ately 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 exam- 

 ined with a magnifier) numerous black grains may be seen. So far as I 

 am able to determine, it belongs to an undescribed species, which I pro- 

 posed to name in honor of Mr. George Gibbs, the geologist of the 

 Boundary Survey. As the matrix in which it is embedded contains no 

 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. 

 alhara from the Tertiary on the Columbia Eiver (Am. Jour. Sci., 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. 



