horizons a much larger size is attained as may be seen by the follow- 

 ing measurements : 



Specimen of average size Harrison Lower Mercer 



Length 14.5 mm. 29 mm. 



Height 6 mm. 13.6 mm. 



Thickness 3 mm. 6 mm. 



Horizon and locality. Harrison ore: Jackson County (Locality 

 1), a. Boggs limestone: Blunt Run, Muskingum County (Locality 

 27), r. Abundant in Lower Mercer, Me Arthur and other members. 



Parallelodon sangamonensis (Worthen) 



1890 Macrodon sangamonensis. Worthen, Geol. Surv. 111., Vol. 8, p. 123, PI. 21, 

 Figs. 3,3a. 

 Coal Measures: Sangamon County, Illinois. 



This form is rare in the Pottsville formation of Ohio, and when 

 found is only about one-half the size of the Illinois specimens, an 

 average-sized individual measuring: length 24 mm., height from beak 

 to ventral margin 13 mm. Extending from the beak to the posterior 

 margin are generally six strong, rounded ribs; the remainder of the 

 shell is smooth or marked by very faint, fine, radiating lines. The 

 general contour resembles Worthen's species so closely that the identi- 

 fication is made with considerable confidence. 



Horizon and locality. Lowellville limestone: Lowellville, Mahon- 

 ing County (Locality 22), r. Also found in the Lower Mercer lime- 

 stone. 



Genus Aviculopinna Meek 

 Aviculopinna americana Meek 



1866 Aviculopinna pinnaeformis. Geinitz, Carb. und Dyas in Nebr., p. 31, tab. 2, 



Fig. 13, (Not Solen pinnaeformis Geinitz, 1848). 

 Nebraska City, Nebraska. 



1867 Aviculopinna americana. Meek, Am. Jour. Sci., 2nd Ser., Vol. 44, p. 282. 

 1872 Aviculopinna americana. Meek, U. S. Geol. Surv. Nebr., p. 197, PI. 9, Figs. 



12a-d. 



Upper Coal Measures: Nebraska City, Nebraska; Iowa. 



Description. Internal casts belonging to the above species are 

 not uncommon in the Boggs member of Muskingum County; the 

 species also occurs more rarely in the Lower Mercer limestone along 

 Flint Ridge. On. the average these specimens are quite comparable 

 in size and contour to the one figured by Meek from the Lower Coal 

 Measures of this State. l However, some of the forms from the Boggs 

 limestone are much larger, the largest individual among the collection 

 studied measuring, if restored, about 3.25 inches (or 82 mm.). As 

 nearly as can be judged from the crushed condition of the shells, 

 the convexity was originally great and the valves were flattened just 



iMeek, F. B., Pal. Ohio, Vol. II, p. 337, pi. 20, Fig. 2, 1875. 



70 



