DESCRIPTION OP SPECIES 179 



member measures: length 6.2 mm., width 10.2 mm., convexity 1 mm. 

 The measurements of another specimen from the Boggs limestone 

 where the form is on the average larger than those of the Lowellville 

 limestone are: length 8.5 mm., width 12 mm., convexity 2 mm. 



Remarks. In the Boggs member this species occurs most com- 

 monly as internal casts on which the muscular impressions and other 

 internal markings are often well preserved. The Ohio forms differ 

 from the typical C. choteauensis from Oklahoma in being less convex, 

 with the mesial sinus or flattening correspondingly less pronounced. 

 These variations, however, may be attributed largely to compression. 

 It is abundant in the Lowellville and Boggs members, and has been 

 found at a few localities in the Lower Mercer limestone; in the first- 

 mentioned member it constitutes the most characteristic fossil. 



Horizon and locality. Lowellville member: Muskingum County, 

 Localities 19 (a), 20 (c), 21 (c); Mahoning County: Locality 22, a. 

 Boggs limestone: Muskingum County, Localities 26 (a), 27 (a), 28 (c), 

 29 (c). Lower Mercer limestone: Scioto County, Locality 31, c; Jack- 

 son County, Locality 32, r. 



Chonetes mesolobus Norwood and Pratten 

 PI. VII, figs. 14, 15 



1855 Chonetes mesoloba. Norwood and Pratten, Jour. Acad. Nat. Sci. Phil., 2nd Ser., 

 Vol 3, p. 27, PI. 2, Figs. 7a-c. 

 Coal Measures: Belleville, Illinois; Charboniere, Missouri. 



Remarks. This common and characteristic fossil of wide distri- 

 bution throughout the Pennsylvanian rocks of the United States, is not 

 found in the lower Pottsville formation of Ohio, but is common and 

 universally distributed in the members including and above the Lower 

 Mercer limestone. It is, however, particularly characteristic of the 

 Lower Mercer horizon in which it occurs in extreme abundance. Chonetes 

 choteauensis characterizes the Lowellville and Boggs members, while 

 C. mesolobus has not been found on either horizon, with the exception 

 of a few individuals from the Boggs limestone at a single locality. The 

 former species seems to have disappeared almost entirely from the 

 Pottsville sea by the advent of Lower Mercer time, for it has been dis- 

 covered practically at only one locality in Scioto County. In 

 its place C. mesolobus appears abundant and wide-spread, persisting 

 throughout the remainder of the Pottsville and the Allegheny forma- 

 tions. This little shell is unusually interesting because it often retains 

 color markings. The colors occur in rather broad, concentric bands 

 which become narrower and more closely arranged toward the sides 

 and front. 



