DESCRIPTION OF SPECIES 173 



to the beak. However, as pointed out by Girty, individuals vary con- 

 siderably in regard to the elevation and position of the beak. 1 



Dimensions. A specimen of average size measures: diameter 6.5 

 mm., convexity 2.5 mm. 



Horizon and locality. Lowellville member: Muskingum County, 

 Locality 19. c. feoggs member: Muskingum County, Locality 27, r. 

 Lower Mercer member: Licking County, Localities 46, 49, r. Upper 

 Mercer member: Columbiana County, Locality 78, r. McArthur mem- 

 ber: Vinton County, Locality 84, r. 



Orbiculoidea stoutella n. sp. 

 PL VII, figs. 5-8 



Description. Shell large for specimens of this genus, subcircular 

 in outline, gibbous, width slightly greater than length. Ventral valve 

 unknown. Dorsal valve with widest portion a little anterior to middle 

 of shell; anterior margin broadly rounded, posterior margin more nar- 

 rowly rounded; greatest convexity about the middle of the valve; 

 beak small, very inconspicuous and depressed, situated almost at the 

 posterior margin of the shell; area posterior to the beak sloping to the 

 margin with a concave outline. Epidermis of shell where preserved, 

 thick, with surface marked by fine, closely arranged, irregular, con- 

 centric lines of growth. 



On the specimens at hand, which are for the most part casts of the 

 interior, not only are internal markings of especial interest retained, 

 but they are preserved in an unusual manner. A well-marked rhom- 

 bohedral or lozenge-shaped visceral area is present in the posterior 

 portion, extending anteriorly about one-third or more of the length of 

 the shell. The posterior margin of this area includes the beak and is 

 sharply marked, but is less well defined laterally and anteriorly. The 

 anterior edge of the lozenge-shaped area is bordered by two shallow 

 rather faintly defined grooves, one on either side of a low median 

 septum, which extend diagonally upward toward the lateral angles of 

 the area. These grooves were formed by ridges on the inside of the 

 shell, which doubtlessly served for the attachment of muscles, and ap- 

 parently correspond to the anterior muscular ridge of Lindstroemella 

 aspidium as figured by Hall and Clarke. 2 On either side of the median 

 septum and just above the muscular ridge is a sharply defined, large, 

 elevated (therefore deeply impressed on the interior of the shell) mus- 

 cular scar; while closer to the septum and on either side of it, is an irregu- 

 ular, less well defined area of muscular attachments which probably 

 represent the anterior adductor muscular scars. The writer has been 

 unable to work out the function of the muscles which produced the 



Girty, G. H., U. S. Geol. Surv., Bull. 436, pp. 22-24, PI. 1, Figs. 6-10, 1910. 

 and Clarke, Pal. New York, Vol. 8, Pt. 1, PI. 4E, Figs. 25, 26, 1892. 



