584 PALEONTOLOGY OF ILLINOIS. 



Position and locality La Salle, Illinois; Silicious Limestone of the 

 upper division of the Coal Measures. 



Genus CLIM)PISTHA, M. and W. 

 CLINOPISTHA KADIATA, VAE. LEVIS, M. and W. 



PL 27. Fie 7. 



Clinopistlia radiata, var. Icvis, MEEK ami WOUTIIEX. Proceed. Aead. Nat. Sci., Phila., 1870, p. -14. 

 Edmondia radiata, HALL, 1856. Iowa Geol. Report, Vol. I, Part 2, p. 716 ; PL 29, Fig. 3. 



SHELL oval, approaching oblong, the hight being from 

 two-thirds to three-fourths the length, moderately convex 

 in young examples, and becoming ventricose with age ; an- 

 terior side much longer and wider than the others, regularly 

 rounded in outline; posterior side very short, rather nar- 

 row below the beaks, and vertically truncated; ventral mar- 

 gin most convex a little in advance of the middle, thence 

 round upward into the front, while behind the middle, or 

 nearly under the beaks, it is slightly contracted or sinuous ; 

 dorsal outline sub-parallel to the base, the margins of the 

 valves being erect anteriorly and rounding into the anterior 

 margin; hinge line rather short; beaks rather .ventricose, 

 rising above the hinge line and placed about half way be- 

 tween the middle and posterior end; posterior umbonal 

 slopes often rendered somewhat prominently rounded by a 

 slight flattening of the valves behind the beaks, near the 

 truncated margin ; ligament lance-oval in form, rather 

 short, and placed immediately behind the beaks, exactly in 

 the position we would expect to see the lunule if the shorter 

 side of the valve were the anterior ; surface with a polished 

 appearance, and generally only showing fine lines of growth, 

 but in some examples also having obscure radiating marks 

 near the ventral margin, which are nearly always denned 

 on internal casts. 



Length of a large specimen, 1 inch; hight, 0.62 inch; 

 convexity, 0.51 inch. 



