302 PALEONTOLOGY OF ILLINOIS. 



prominent part of its posterior extremity above, instead of below the middle. 

 Its concentric strhe are also stronger and more regular. The specimen we have 

 figured is an internal cast, retaining only very obscure traces of these strise; 

 but others before us have some remaining portions of the shell, on which they 

 are distinctly visible, particularly on the anterior side of the umbones, where 

 they seem to have been most strongly marked. It has more nearly the form 

 of a shell figured by McCoy (Carb. Foss. of Ireland, pi. 8, fig. 6), under the 

 name Anatina attenuate/,, but has its beaks more elevated. The form figured 

 by Prof. McCoy is probably a Schizodus, at any rate it is almost beyond doubt 

 not an Anatina. 



Locality and position : Chester, Illinois ; Upper part of Chester group, of 

 Lower Carboniferous series. 



GASTEROPODA. 



Genus STRAPAROLLUS, Montfort, 1810. (See page 158.) 



(Conch. Syst., ii, p. 174.) 



Straparolltjs planidorsatus, M. and W. 



PI. 24, fig. la, 2b, 2e. 



Euomphalus planidorgatus, Meek and Worthen, Oct., 1860. Proceed. Acad. Nat. ScL, 

 Philad., p. 462. 



Shell of medium size, subdiscoidal ; spire nearly flat, or but 

 slightly elevated above the body whorl ; volutions about five, 

 increasing gradually in size, flat above, and provided with a 

 moderately distinct revolving angle about two-thirds of the 

 distance across from the inside. From this angle the last whorl 

 is regularly rounded over the periphery to about the middle of 

 the under side, where there is another angle, from which the 

 inner side of the whorl rounds into the umbilicus. Suture 

 well defined. Umbilicus large, deep, and showing about one- 

 half of each inner whorl ; aperture subcircular. Surface often 

 scarcely showing any striae, but in some instances with obscure 

 lines of growth most distinctly seen in the umbilicus. Breadth, 

 1 inch; height, about 0.30 inch. 



