98 WINCIIELL AND MARCY ON FOSSILS FROM THE 



PLEUROTOMARIA Defrance. 



Pleurotomaria gonopleura W. and M. 



Plate III. figure 4. 



Shell conical, somewhat flattened on the upper portion of the whorls and on the base. 

 Number of whorls six or seven, enlarging with regularity ; suture moderately impressed ; 

 keel near the lower side of the whorl, quite prominent; whorl rounded below this, to the 

 suture ; above the keel, nearly half way to the suture is another low ridge separated from 

 the first by a concave belt ; above this ridge the whorls are flattened convex. Surface 

 marked only by incremental lines which incline backwards in passing from the suture to the 

 keel, and continue in the same direction into the umbilicus, suffering a slight retral inflec- 

 tion in passing the peripheral band. 



Height, 1.19 inch ; diameter of base, .98 inch ; height of last whorl, .50 inch. 



This species, though resembling P. Hoyi Hall, from the Racine limestone, has two or three 

 more volutions ; and has the band on the lower, instead of the upper, angle of the whorls. 



Pleurotomaria Halei Hall? Wis. Rep. 1861, p. 34. Abundant specimens of a Pleurotoma- 

 ria occur, which agree generally with the above species. They differ as follows : Height 

 equal to transverse diameter, instead of two thirds that diameter; number of volutions 

 five or six instead of three or four; number of revolving ridges on the upper side of the 

 volution five or six, instead of ten or twelve. The periphery, moreover, can hardly be said 

 to be sub-angulated. 



To Hall's description it may be added, that the under side of the body whorl is marked 

 by about thirteen wavy, revolving ridges, alternately larger and smaller, intersected by 

 transverse striae emerging from the umbilicus. It resembles P. decussata Sandberger, from 

 the stringocephalenkalk of Nassau. 



We are in possession of the casts of the umbilical cavity of a very large Pleurotomaria 

 which may belong to this species. The revolving ridges rise obliquely from the umbilical 

 cavity, and are intercepted nearly at right angles by a set of less numerous ridges, also ris- 

 ing from the umbilicus, and curving in the opposite direction. Some of these casts show 

 that the umbilical cavity exposed five whorls. The diameter of the base of the largest 

 individual must have been two and a half inches. This individual was reversed, or sinis- 

 tral. It is not unlikely that the great size of the umbilicus in these casts, and the inter- 

 rupted character of the ridges emerging from it, and perhaps also the great size of the speci- 

 mens, give indications of an undescribed species. 



Pleurotomaria Hoyi Hall. Wis. Geol. Rep. 1861, p. 35. No flattening appears on the lower 

 side of any of the volutions of our specimens. 



Pleurotomaria sigaretoides W. and M. 

 Plate III. figure 5. 



Shell small, depressed-conical, oblique, sinistral (in our single specimen), consisting of three 

 or four rapidly enlarging whorls. The whorls are flattened on the exterior, leaving a linear 

 suture. The last whorl is two and a half times as broad on the flattened exterior as the 

 penultimate whorl, sharply angulated at the periphery, which forms the outer boundary of 

 the flat, or shallow funnel-shaped base. It is supposed that this angle is the place of the 



