62 University Geological Survey of Kansas. 



Derbya crassa. Plate VIII, figs. 11, lib. 



Orthis arachnoides Roemer (non Phillips), Kreidebildung Texas, p. 89, 



pi. xi, f. 9, (1852); etc. 

 Orthisina crassa Meek and Hayden, Proc. Acad. Nat. Sci. Phil. 1858, p. 261. 

 Orthis lasallensis McChesnev, Desc. New Pal. Foss., p. 32, (1860); ibid., 



pi. i, f. 6, (1865). 

 Orthis richmonda McChesney, ibid., p. 32; also pi. i, f. 5. 

 Hemipronitcs crassus Meek and HaydeD, Pal. Upp. Mo., Smiths. Cont. 



Knowl., xiv, 172, p. 26, pi. i, f. 7, (1864); Meek, Fin. Rep. U. S. Geol. 



Surv. Neb., p. 174, pi. v, f. 10, pi. vin, f. 1, (1872); etc. 

 Orthis creni stria Geinitz (non Phillips), Carb. u. Dyas in Neb., p. 46, 



pi. m, ff. 20,21, (1866). 

 Hemipronites lasallensis McChesney, Trans. Chic. Acad. Sci., i, p. 28, 



pi. i, f. 6, (1868). 

 Hemipronites richmonda McChesney, ibi'd., p. 28, pi. i, f. 5. 

 Hemipronites crenistria White, Wheeler's Expl. Surv. West. 100 Mer., 



iv, p. 124, pi. x, f. 9, (1875). 

 Streptorhynchus richmondi Hall, 2d Ann. Rep. N. Y. St. Geol., pi. xl, 



ff. 10, 11, (1883). 

 Derbya crassa Waagen, Pal. Indica. ser. xin, i, p. 592, (1884); etc. 



Meek's description (in part) : " Shell varying from nearly cir- 

 cular to truncato-subcircular, or transversely suboblong, generally 

 wider than long, varying from compressed to distinctly convex ; 

 hinge margin equaling or shorter than the greatest breadth of 

 the valves, rectangular, or sometimes more or rather less than 

 rectangular, at the extremities ; anterior outline forming a more 

 or less regular semicircular curve. Dorsal valve always convex, 

 sometimes very distinctly so, the greatest convexity being near 

 the middle ; beak not distinct from the cardinal margin. A^en- 

 tral valve varying in convexity at the umbo, sometimes very 

 prominent, and occasionally distorted there ; less convex, flat- 

 tened, or not unfrequently a little concave, around near the 

 front ; area varying in height in proportion to the elevation to 

 the beak, and either flat or with the beak a little arched, usually 

 rather distinctly striated ; its closed fissure varying in the pro- 

 portions of height and breadth with the greater or less eleva- 

 tion of the beak ; interior always provided with a prominent 

 mesial septum extending from the beak forward to near the 

 middle of the valve ; surface on both valves marked by numer- 

 ous strong, raised radiating stride of unequal size, there being 

 generally one or several smaller ones between each two of the 

 larger ; crossing the whole are also numerous fine, regular con- 



