214 
Kirtley F. Mather 
ilar to that of the material at hand. It is possible that the 
forms from the Morrow group are closer to Stevens’s type than 
many of the specimens which have been thus identified. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Stations 136 and 137). 
PARALLELODONTID^ 
Genus PARALLELODON Meek 
Parallelodon sangamonensis (Worthen) 
1890. Macrodon sangamonensis. Worthen, Geol. Surv. 111., vol. 8, p. 123, 
pi. 21, figs. 3-3a. 
Coal Measures: Roll’s Ford, Sangamon County, Illinois. 
1894. Macrodon sangamonensis ?. Keyes, Mo. Geol. Surv., vol. 5, p. 121, 
pi. 46, fig. 2. 
Upper Coal Measures: Kansas City, Missouri. 
1900. Macrodon sangamonensis ?. Beede, Univ. Geol. Surv. Kans., vol. 
6, p. 146, pi. 20, figs. 2-2b. 
Upper Coal Measures: Turner, Wyandotte County, Kansas. 
1911. Parallelodon sangamonensis. Mark, Bull. Sci. Lab. Den. Univ., 
vol. 16, p. 310, pi. 9, fig. 10. 
Mercer limestone: Bald Knob, Ohio. 
An imperfect internal cast from the Kessler limestone is 
referred with considerable confidence to this species. The strong 
radiating costae over the posterior portion of the shell, which 
become successively weaker anteriorly, and the restriction cross- 
ing the valve from umbo to ventral margin with the consequent 
slight sinuosity in the latter, are well shown. The shell was 
probably smaller than most of the forms thus identified as its 
length could not have been much over 19 mm. when restored. 
Horizon and locality. Kessler limestone : East Mountain, Fay- 
etteville, Arkansas (Station 209). 
Parallelodon pergibbosus n sp. 
Plate XV, figure 6. 
Description. Shell below medium size, transversely subellip- 
tical in outline, very inequilateral, strongly convex, nearly three 
times as wide as high, hinge-line nearly as long as greatest width 
of valve; cardinal margin straight and met by the anterior 
margin in a right angle, the anterior outline curviiig gracefully 
into the ventral margin which is nearly straight and parallel 
