form from the Humerosa formation of Colorado, described and figured 

 by Girty as S. boonensis? Both are characterized by their long hinge 

 lines which form the greatest width of the shell, and by their coarse 

 ribs of which three to five occupy the sinus and four to six the fold. 

 In the Colorado form there are twelve to thirteen lateral plications, 

 while the form from Ohio has ten to twelve on either side of the fold 

 and sinus. There can be little doubt that the forms under considera- 

 tion belong to S. boonesis as interpreted by Girty. 



Horizon and locality. Boggs limestone: Muskingum County, 

 near Hopewell P.O. (Locality 26), a; Blunt Run (Locality 27), a; 

 Rock Cut (Locality 28), a; Symmes Creek (Locality 29), a. Abundant 

 in the marine limestones above the Boggs member. 



Spirifer opimus Hall 

 PI. Ill, Figs. 17, 18, 19, 20, 21, 22. 



1858 Spirifer opimus. Hall, Geol. Iowa, Vol. 1, Pt. 2, p. 711, PI. 28, Figs, la, b. 



Coal Measures: Ohio, Maryland, Iowa, etc. 

 1915 Spirifer opimus. Mather, Bull. Den. Univ., Vol. XVIII, p. 185, PI. 12, Figs. 



7-7c. 



Morrow r formation : Arkansas and Oklahoma. 



Hall's description: "Shell rotund, gibbous, length and width 

 nearly equal; hinge line equaling or sometimes a little greater or less 

 than the width of the shell below; valves nearly equally gibbous in 

 their greatest convexity. Dorsal valve regularly convex, with a 

 strong, well defined mesial fold which is simple at the apex, dividing 

 a little below, and each division again dicotomizing, the two middle 

 divisions stronger than the lateral ones, and separated by a well- 

 defined groove; in some of the smaller shells, the lateral plications 

 of the mesial fold are feebly or not at all developed: beak elevated 

 a little above the hinge line, and incurving over a narrow defined 

 area. Ventral valve most gibbous above the middle, and abruptly 

 rounding toward the sides and front; mesial sinus well defined, simple 

 above, and becoming marked by three small plications in the middle 

 and lower part; beak much elevated and strongly incurved, covering 

 the upper part of the large foramen; area high in the middle, slightly 

 concave, continued to the extremities of the hinge line, vertically 

 striated; foramen large, forming a equilateral triangle. 



Surface marked by from eight to ten simple abruptly elevated 

 plications (on either side of the fold and sinus) 1 which are equal to 

 the spaces between, concentrically marked by strong imbricating 

 lamellose lines of growth, and, on well-preserved specimens, by finer 

 radiating and concentric striae." 



The Ohio forms are uniformly small with the hinge line equal to 

 or less than the greatest width of the shell below. The sinus of the 

 ventral valve is marked by three to five bifurcating plications, of 



L The words in parenthesis have been applied by the writer. 



57 



