INVERTEBRATE PALEONTOLOGY. 93 



nent, central, very gibbous, and incurved withoul visible obliquity. Hinge- 

 area and surface-markings unknown. 



Length, about 0.26 inch; height, 0.23 inch; convexity, 0.2:5 inch. 



As in the last, this little shell is only known in the condition of internal 

 casts. At one time I thought it might possibly be the young of T. Siouxensis, 

 but a more careful examination shows that it has proportionally more ventri- 

 cose beaks, and that its dorsal-margin declines more directly from the same, 

 instead of being somewhat straightened along the middle, while its denticles 

 are proportionally larger and less numerous. The specimens are very 

 imperfect, only the one figured showing impressions of the hinge-teeth on one 

 side of the beaks. Still, it seems desirable to call attention to it as charac- 

 teristic species of this rock. 



Locality and position. — Twelve miles southwest of Salina, Kansas; in 

 the Dakota group of the Upper Missouri Cretaceous series. Prof. B. F. 

 Mudge, collector. 



Trigouarca (Brc vi arc a) cxigua, M. & H. 



Plate 15, figs. 2, a, 6, c, d, e, f. 



Cuculhm exrigua, Meek aud Hayden (1856), Proceed. Acad. Nat. Sci. Pbilad., VIII, 275. 

 Area exigua, Meek and Hayden (Oct., 1860), ib., XII, 428. 



Shell very small, rather thick, subquadrate or rhombic-cordate, gibbous, 

 equivalve; anterior margin rounded ; posterior side obliquely truncated ; base 

 rounding up regularly in front and abruptly behind, being nearly straight 

 along the middle; ligament-area well defined, equaling a little more than half 

 the length of the shell, provided with a divaricating linear groove, extending 

 from near the beaks to the cardinal margin so as to inclose a triangular space, 

 which is crossed at right angles to the hinge by very regular, closely-arranged, 

 minute parallel striae ; pallial margin smooth within ; beaks prominent, gibbous, 

 rather distant, central, and incurved at right angles to the hinge. Surface 

 marked by a few distant concentric furrows and extremely fine striae, 

 crossed by equally fine, obscure, closely-arranged radiating striae. 



Length, 0.32 inch; height, 0.2G inch; convexity, 0.25 inch. 



The hinge-denticles in this species are slightly bent at their inner (lower) 

 ends, and radiate outward so that those on each side intersect the hinge-line 

 at an angle of about 40°. The hinge is comparatively broad toward the ex- 

 tremities, and narrow in the middle. Internal casts show that the anterior 



