DESCRIPTION OF SPECIES 227 



Dimensions. Individuals vary greatly in size. The largest speci- 

 men examined was obtained from the McArthur limestone and meas- 

 ures: height 40.5 mm., length 35 mm. A large individual from the 

 Lower Mercer limestone measures: height 26 mm., length 23 mm., 

 while the dimensions of a specimen of average size are: height 17 mm., 

 length 14 mm. 



Horizon and locality. Widely distributed throughout the Lower 

 Mercer and McArthur members, c. 



Aviculopecten pellucidus Meek and Worthen 



1860 Aviculopecten pellucidus. Meek and Worthen, Proc. Acad. Nat. Sci. Phil., p. 455. 



Coal Measures: Adams County, Illinois. 

 1866 Aviculopecten pellucidus. Meek and Worthen, Geol. Surv. 111., Vol. 2, p. 327, 



PI. 26, Figs. 5 a, b. 



Lower Coal Measures: Adams County, Illinois. 



Description. A single specimen of this small, delicately sculp- 

 tured Aviculopecten was found in the black shale associated with the 

 Lowellville limestone. Its surface is ornamented by slender ribs, a 

 little more than their own diameter apart, which are crossed by rather 

 strong, more closely arranged concentric lines. Where the radiating 

 and concentric markings cross each other, little nodes or scales are 

 formed. The ornamentation of the ears is similar to that of the body 

 of the shell. 



Dimensions. Height 6.5 mm., length about 6 mm. 



Horizon and locality. Lowellville member: Muskingum County, 

 Poverty Run, Locality 19, r. 



Aviculopecten sorer Herrick 



1887 Aviculopecten sorer. Herrick, Bull. Den. Univ., Vol. 2, p. 27, PI. 1, Fig. 7; PI. 3, 

 Fig. 16. 

 Coal Measures: Flint Ridge, Ohio. 



Description. Rather small aviculoid shells occur in the collec- 

 tions of Lower Mercer fossils from Muskingum and Licking counties, 

 which in all probability represent the same form which Herrick referred 

 to as Aviculopecten sorer, although a number of points of divergence 

 were noted. However, the fact that the specimens under discussion 

 came from the same locality and horizon as Herrick's species lends 

 additional weight to the identification. It is of rather rare occurrence 

 and all of the individuals studied are more or less imperfect. The size 

 is considerably less than Herrick's figures indicate, the largest individ- 

 ual observed being about two-thirds as large as the one found on Plate 

 III, Fig. 16. The ears are subequal with the anterior one convex and 



