INVERTEBRATE PALEONTOLOGY. 529 



rather elevated, pointed, and incurved with a forward obliquity; posterior 

 umbonal slopes having an obscure ridge extending obliquely from the beaks 

 to the postero-basal margin ; surface ornamented by moderately distinct lines 

 of errowth, which are must strongly defined behind the oblique umboual 

 ridge. 



Length. 0.64 inch; height, 54 inch: breadth, about 0.34 inch 



The tooth of the right valve is rather small, obtuse, and located imme- 

 diately under the point of the beak, while the process in the other valve is 

 oblique, compressed, and bears on its upper surface a distinct scar, left by 

 the attachment of the cartilage. The pit in the opposite valve for the recep- 

 tion of the cartilage, and this compressed process of the left valve, is com- 

 paratively large, trigonal, and deep. Just in front of the tooth in the right 

 valve, and along the entire length of its posterior cardinal margin, there is a 

 distinct groove for the reception of the sharp cardinal edge of the other valve. 

 A similar, but less distinctly-marked groove is seen a little within the basal 

 margin of the right valve, which groove is occupied by the edge of the left, 

 when the valves are closed. The muscular impressions are faintly marked, 

 though I have been able to see that the anterior one is narrow-ovate, 

 and the posterior larger and proportionally broader. The pallial line is pro- 

 vided with a broad shallow sinus. 



I am aware that the name mactriformis, at first inadvertently selected 

 for this species, is faulty in construction; but I do not think it a matter of 

 sufficient importance, even if it were quite clear that the rules of nomen- 

 clature would permit such a change, to attempt the substitution of a less 

 objectionable name now. 



Locality and position. — Fort Clark, Dakota, on the Upper Missouri; 

 from the Fresh- or Brackish-water Lignite beds of that region: probably 

 Lower Eocene. 



C o r b it I a subtrigoaalis, M. & H. 



Plate 40, tigs. 3, a, b. 



Corbulu subtrigonalis, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 116. 

 Corbula (Potamomya) subtrigonalis, Meek and Hayden (I860), ib., XII, 432.— Meek (1866), in Conrad's 

 Smithsonian Check-List N. Am. Eocene Invert. Fossils, 8. 



Left valve longer than high, trigonal-ovate, very convex ; anterior side 

 rather abruptly rounded below; base semi-ovate, the most convex part being 

 near the front; posterior side longer than the other, and angular, or very 

 r.T it 



