GA.STEEOPODS. 207 



to twenty-five on the last volution, while there are from three 

 to four less on each preceding turn. 



Horizon and localities. Upper Carboniferous, Upper Coal 

 Measures: Kansas City. 



There have been described from the Coal Measures of 

 the Mississippi basin, a number of forms similar to the one 

 under consideration. At firnt glance the vertical costse appear 

 perfectly straight, instead of arched, as in the more typical 

 shells of the genus. Upon a closer examination, however, 

 these ribs are found to curve considerably. The relatively 

 much larger size of the rounded ridges in this section of the 

 genus, as compared with the Devonian forms, tends greatly to 

 obscure the actual curvature. 



The figures accompanying the reprint of the original de- 

 scription are somewhat misleading in at least one particular : 

 that is, in having the costie of the adjoining whorls arranged 

 directly above one another. The statement is also made that 

 such is the case. Closer observation clearly shows that the 

 arrangement alluded to is more apparent than real ; and that 

 in reality the ridges of the different turns are set slightly behind 

 those of the preceding volution. Instead, therefore, of form- 

 ing perfectly straight though interrupted folds, running from 

 the middle of the body-whorl to the apex of the shell, the long 

 rounded elevations are more or less twisted spirally to the 

 right. 



The number of costse varies with the size of the shell. 

 Usually there are from three to four ribs less on each whorl 

 than on the one immediately preceding. Thus, the eighth whorl 

 may have from twenty to twenty-five or more rounded ridges, 

 while the fourth has only a dozen or fourteen. Below the 

 middle of the volutions the costae rapidly dimmish both in 

 height and breadth, and are continued as minute hair-lines. 



The aperture is oval in outline, slightly flattened on the 

 inner side. The lip springs abruptly from the callous portion, 

 instead of gradually blending with it at a low angle, but other- 

 wise the shell possesses all the characteristics of the typical 

 forms of Loxonema. 



