21 



This shell is distinguished from either of the preceding 

 by its slender volutions which increase much more gradually 

 from the apex. The volutions are round both above and 

 below, though sometimes the lower side descends so abrupt- 

 ly to the umbilicus as to present the appearance of un ob- 

 tuse or undefined angle on the last volution. 



Pleurotomaria xodulostriata. — Shell turbinate ; spire 

 depressed-conical, obtuse at the apex ; volutions about four, 

 rounded, somewhat depressed above, the last one ventricose 

 below j suture distinct, rather sharply defined ; aperture 

 sub-circular, slightly flattened on the inner side ; umbilicus 

 rudimentary ; surface marked by strong, revolving, elevated 

 striae which are about equal to the spaces between them, 

 excepting on the periphery of the outer volution where two 

 or three are more distant, leaving a double spiral band ; re- 

 volving striae crossed by oblique striae (parallel to the lines 

 of growth) which are very conspicuous on the upper side 

 of the volution, but become obsolete below the band. The 

 revolving lines at the junction of the oblique striae become 

 nodulose on the upper half of the volution, and particularly 

 near the suture. 



Diameter "12 to *18 ; height *10 to *18 of an inch. 

 Locality. — Spergen Hill ; Bloomington. 



, Pleurotomaria humilis. — Shell depressed, trochiform, 

 oblique, spire little elevated, consisting of three or four vo- 

 lutions which increase rapidly in size from the apex ; volu- 

 tions depressed-convex above, and declining to the periph- 

 ery ; base of the last volution less convex than on the up- 

 per side, subobtusely angular on the periphery which is 

 marked by a narrow groove, little wider than the usual spaces 

 between the revolving striae ; surface marked by revolving 

 and transverse striae which are stronger and more distant 

 on the upper side of the volution, giving it a beautiful can- 



PLEUROTOMARIA, Defrance. 



