56 University Geological Survey of Kansas. 



face, which is nearly fiat, but it is depressed above the foramen, 

 which is of the usual character ; the foramen is situated just 

 beneath the beak of the upper valve." The posterior portion 

 of the shell of one individual which has the outer portion of 

 the shell exfoliated shows distinct, discontinuous, alternating, 

 radiating ridges, from near the beak to the posterior margin. 

 " Diameter of the upper valve just described, 27 mm. ; height, 

 12 mm." 



Range and distribution : Upper Coal Measures ; Kansas City, 

 Topeka. 



Orbiculoidea manhattanensis. Plate VIII, figs. 2-2b. 



Discina manhattanensis Meek and Hayden, Proc. Acad. Nat. Phil., 



(1859), p. 25. 

 Orbiculoidea manliaitanensis Hall and Clarke, Introduction to Studv 



of Brach., pt. i, pi. v, f. 12, (1892): Pal. N. Y., vm, pt. i, pi. iv, f. 20, 



(1892). 



Shell of medium size for this genus, nearly circular, dorsal 

 valve moderately elevated ; umbo situated about one-half of a 

 diameter from the posterior margin ; surface of dorsal valve 

 marked with concentric lines, which are much heavier near the 

 margin and finer and more wavy near the apex. The anterior 

 slope is slightly convex, posterior a little concave, distinctly so 

 in casts. The ventral valve is flat, shell quite thick ; sulcus in 

 distinct depression, extending about half way from the center 

 to the margin ; opening linear-elliptical. There is a thick 

 callosity near the outer margin which is marked by radiating 

 ridges corresponding to those of the other valve. Diameter of 

 large specimen, 12 mm. ; height, 2i mm. Outer surface 

 marked by concentric lines similar to the convex valve. 



Range and distribution : Upper Coal Measures ; Wabaunsee 

 formation ; black shale in wagon-road cut east side of Blue 

 Mount, Manhattan, Kan. In collection of E. A. Popenoe. 



This species differs from 0. missouriensis in being more com- 

 pressed, thicker shelled, shorter sulcus, and in having more 

 distinct concentric lines ; from 0. eonvexa, in being much less 

 convex and much smaller. 



