338 PALAEONTOLOGY OF ILLINOIS. 



Eumickotis Hawni var. sinuata, M. and W. 



PI. 27, figs. 12, 12 a, 12 6, 13 and 14. 



Monotis Hawni, Meek and Hayden. Trans. Albany Inst., iv, March 2, 1858. 

 Eumicrotis Hawni, Meek. Am. Jour. Sci., xxxvi, March, 1864, p. 216; Meek and Hay- 

 den, Palreont. Upper Missouri, 1865, p. 54. 



Shell oval-suborbicular, plano-convex, not oblique; ventral 

 margin rounded in outline; posterior side obliquely truncated 

 above ; anterior side somewhat obliquely straightened above, 

 or rounding regularly from below the ear to the base ; cardinal 

 margin shorter than the greatest transverse diameter of the 

 valves; hinge area moderately broad, and distinctly inclined 

 over towards the left valve; cartilage pit distinct, rather broad, 

 and placed directly under the beaks, being common to both 

 valves. Left valve ventricose, umbo not oblique, incurved and 

 projecting a little above the cardinal margin ; posterior ear 

 little developed and obliquely truncated ; anterior ear slightly 

 longer, rounded at the extremity, and separated from the ante- 

 rior margin by a deep, well defined, angular sinus; surface 

 ornamented with more or less waved or distorted radiating 

 costae, of unequal size, there being generally one, two or three 

 smaller ones between each two of the larger — all crossed by 

 concentric, imbricating lamillae of growth, which on the larger 

 ribs rise in the form of vaulted subspiniferous scales. Right 

 valve flat, or very nearly so ; beak obsolete ; byssal sinus very 

 narrow and deep ; muscular impression subovate, oblique and 

 placed a little behind the middle; surface much more obscurely 

 costate than that of the left valve. Diameter from hinge to 

 basal margin, 1.25 inch; antero-posterior diameter, 1.12 inches; 

 convexity, about 0.35 inch. 



Although we have placed this as a variety of E. Hawni, we are much inclined 

 to believe it will prove to be a distinct species, since it differs from all the 

 Kansas specimens of that shell we have seen, in having a rather distinct sinus, 

 in the anterior margin of the left valve, as well as in the truncated outline of 



