598 PALAEONTOLOGY OF ILLINOIS. 



Genus MICKODOMA, M. and W. 

 MICKODOMA CONICA, M. and W. 



PL 28, Fig. 2. 

 Microdoma eonica, MEEK and WOUTIIEN, 1866. Proceed. Acad. Nat. Sci., Phila., p. 269. 



SHELL rather elongate-conical, or elevated subtrochi- 

 form ; volutions seven, flattened on a line with the slope of 

 the spire, increasing rather gradually in size, last one ex- 

 tended a little below the mesial angle, where it is only 

 marked by minute strisB of growth; suture rather deep; 

 aperture a little oblique, and oval-subrhombic in outline. 

 Surface ornamented by three distinct revolving nodular 

 ridges, the largest and lowest of which occupies the mesial 

 angle of the body whorl, and passes around immediately 

 above the suture of the whorls of the spire, while the 

 upper one occupies the upper margin of all the whorls 

 just below the suture, and the third one passes around 

 midway between the others. Lines of growth small, and 

 crossing the flattened sloping sides of each whorl so ob- 

 liquely as to indicate a distinct forward extension of the 

 outer lip above, at its connection with the body whorl. 

 Nodes of the revolving ridges small, closely and regularly 

 arranged on the different ridges, so as to form oblique rows 

 parallel to the lines of growth. 



Length, 0.21 inch; breadth, 0.12 inch; apical angle with 

 straight slopes; divergence, 30 deg. 



This species seems to be nearly related to Pleurotomaria aerriliinha^ 

 of Phillips, judging from his figure, but the description of that shell is 

 so very brief and unsatisfactory that we have no means of making a 

 close comparison. 



Locality and position Lower part of Coal Measures ; Macoupin Co., 

 Illinois. 



