DESCRIPTION OF SPECIES 229 



maximum width of the shell below; cardinal area broad with large, tri- 

 angular ligamental pit beneath the beaks. Ventral and anterior mar- 

 gins broadly rounded, posterior margin slightly produced below the 

 middle of the shell; ears subequal, sharply defined on either side of the 

 beaks by a sinus which is deeper on the anterior ear than on the pos- 

 terior; anterior ear slightly convex in outline, posterior ear mucronate 

 at hinge line, strongly concave in outline. Surface marked by fas- 

 cicles of strongly elevated ribs, three to five in each bundle (generally 

 four) which increase by bifurcation; ears also marked by strong radia- 

 ting ribs; entire surface including the ears crossed by fine, regular, 

 closely arranged, concentric lines, which form minute scales where 

 they cross the radiating ribs. 



Dimensions. An individual of average size measures: length 34 

 mm., height 33 mm., length of hinge line 22.5 mm., convexity of left 

 valve 5 mm. 



Remarks. Aviculopecten fasciculatus Keyes and A. providencesis 

 Cox are Pennsylvanian species which also have the fasciculate group- 

 ing of the ribs and to which the Ohio Potts ville form can be compared. 

 The former is a much larger species, almost three times the size of 

 our form, and comes from a much higher horizon in the Pennsylvanian 

 system. A. providencesis is likewise a much larger form. If these 

 species could be reduced to the size of D. scalaris, the ribs would prob- 

 ably be larger and less numerous. 



Horizon and locality. Boggs member: Muskingum County, Lo- 

 cality 26, c. Lower Mercer member: Scioto County, Locality 31, c; 

 Licking County, Localities 46, 47, c; Mahoning County, Locality 55, c. 

 Upper Mercer member: Muskingum County, 68, r. McArthur member: 

 Jackson County, Locality 80, r; Vinton County, Locality 84, c. 



Genus Acanthopecten Girty 

 Acanthopecten carboniferous (Stevens) 



1858 Pecten carboniferous. Stevens, Am. Jour. Sci. and Arts, Vol. XXV, p. 261. 

 Coal Measures: Crooked Creek, Marion County, Illinois. 



Description. Acanthopecten carboniferous is an abundant and 

 widely distributed fossil in the Pottsville formation of this State above 

 and including the Boggs member. It is recognized by its large ears, 

 and by its distinctive surface sculpture of fifteen to eighteen large, 

 regular, plications which are marked at regular, somewhat distant in- 

 tervals by spine-like projections where concentric lines cross the plica- 

 tions. The size is comparatively constant; an individual of average 

 size measures: height 16 mm., length of hinge line 13 mm., greatest 

 width below hinge line 18 mm. A specimen of unusual size from the 



