INVERTEBRATE PALAEONTOLOGY. 247 



dues nol entirely fill the pii under the beak of the other valve, but leaves 

 space enough for an internal cartilage, as in Corbula. 



Nothing is known in regard to the geological range of this genus beyond 

 the fact that the typical and only known species occurs in the Upper Creta- 

 ceous. 



Corbnlamella gregaria, M. & H. 



Plate 17, figs. 13, a, b, c, d. 



Corbula? gre.garia, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 84. 

 Corbulamclla gregaria, Meek and Hayden (1857), ib., IX, 143. 



Shell very small, globose-subtrigonal ; anterior side rounded below ; 

 posterior side subtruncated at the extremity ; base rather broadly rounded, 

 sometimes a little more prominent toward the front than behind ; dorsum 

 sloping abruptly from the beaks in front and rear ; beaks prominent, gibbous, 

 and subcentral, that of the right valve rising distinctly above the other; both 

 incurved nearly or quite at right angles to the hinge. Surface smooth, or 

 only marked by very obscure lines of growth, and a few faint, irregular, 

 undulations. 



Length and height, each 0.13 inch ; convexity, 0.10 inch. 



The inequality of the two valves consists not merely in the more elevated 

 beak and more gibbous form of the right valve, but its pallial border also 

 extends beyond that of the other valve. The tooth, or cartilage-process, of 

 the left valve is larger and more prominent than in the right. The anterior 

 muscular impression is narrow-ovate, faintly marked, and placed near the 

 border; the posterior impression has the same form, and appears to occupy 

 nearly or quite the whole surface of the projecting lamina, which is sharp on 

 its free edge, and a little concave or spoon-shaped. The sinus of the pallial 

 impression is quite shallow, and apparently a little angular. 



Locality and position. — Occurs in great numbers on the Yellowstone 

 River, Montana Territory ; in beds containing a mingling of the fossils of the 

 Fort Pierre and Fox Hills groups, or formations No. 4 and No. 5 of the Upper 

 Missouri Cretaceous series. 



