WAVEELY GEOUP SPECIES. 



Subgenus TRIGONOTRETA, Kin^r, 1825. 

 Spieifee (Teigonoteeta) steiatifoemis, Meek. 



Plate 14, flgs. 80, 6, c, d, e. 



Shell attaining a full medium size, moderately gibbous, semi-oval, or 

 sub-trigonal in outline; hinge nearly or quite equaling the greatest 

 breadth ; anterior margin more or less rounded, or sometimes a little 

 projecting or subangular at the middle ; lateral margins rounding to the 

 front, or sometimes converging forward, with somewhat straightened 

 outlines, and meeting the hinge behind generally at about right angles. 

 Ventral valve more convex than the other, its greatest convexity being 

 near or a. little behind the central region, from which it rounds over a 

 little more abruptly to the beak than to the front and anterior lateral 

 margins ; beak rather pointed, moderately prominent, and strongly in- 

 curved; cardinal area of medium size, well defined, and more or less 

 arched and inclined backward ; foramen about one-fourth to one-fifth as 

 wide as the hinge, and apparently always wider than high; mesial sinus 

 narrow, rather deep anteriorly, and continued much attenuated usually 

 to the beak ; interior (as shown by internal casts) with dental laminae 

 short and diverging, and rostral cavity comparatively rather small, of 

 moderate depth, strongly striated longitudinally, and marked by a few 

 diverging ridges parallel to its anterior lateral margins, while the bot- 

 tom of the valve on each side, in the umbonal region, is usually occupied 

 by numerous small pits. Dorsal valve depressed convex, or sometimes 

 rather prominent at the middle anteriorly, and rounding abruptly to the 

 beak, and less strongly rounded or sloping to the anterior lateral mar- 

 gins ; beak projecting little beyond the hinge, and, with the very nar- 

 row area, distinctly incurved; mesial fold narrow, rising very little 

 above the general convexity in the umbonal region, and continued 

 rounded, without distinctly defined margins, either much depressed or 

 moderately elevated, to the front, where it is sometimes prominent 

 enough to give origin to a semicircular sinus in the edge, to receive a 

 corresponding short projection of the margin of the other valve. Sur- 

 face of both valves (including sinus and fold) ornamented by numerous, 

 rather small, depressed, rounded, more or less bifurcating, longitudinal 

 costse, crossed toward the front and lateral margins by distinct, sub-im- 

 bricating marks of growth, while over the whole, well-preserved speci- 

 mens show a very beautiful, minute, cancellated sculpturing, formed by 

 numerous, very sharply defined, and crowded radiating and concentric 

 striae. 



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