392 REP OUT ON THE STATE CAB I WET. 



GENUS PLEUROTOMARIA, De France. 

 Pleurotomaria occidens, n. s. 



PLATE XV, FIGS. 11, 12; PLATE XXV, FIGS. 9, 10. 

 Pleurotomaria labrosa, -var. occidens. Twentieth Rep. N. Y. St. Cab., 1st edit., p. 343. 1867. 



Shell somewhat rhomboidal-ovate ; spire moderately elevated ; volutions 

 about three, the last one rapidly expanding, subangular, and marked 

 by a spiral band a little above the middle ; upper side somewhat 

 flattened; lower side I'ounded, and in the last one becoming ventri- 

 cose. Surface marked by strong revolving and transverse striae. 



An examination of other specimens of this species proves it to be dis- 

 tinct from the P. labrosa to which I had heretofore referred it as a variety. 



Pleurotomaria halei, Hall. 



PLATE XV, FIGS. 13, 14. 

 Pleurotomaria haki, Hall. Report of Progress Geol. Survey Wisconsin for 1860, p. 34. 1861. 



Shell suborbicular, the spire moderately ascending, the height equal to a 

 little more than two-thirds the greatest diameter, consisting of three 

 or four volutions which are rounded on the top, and expanding 

 somewhat rapidly in size ; the last one quite ventricose, and in the 

 cast is subangular on the periphery ; the under side of the last volu- 

 tion is rounded from the edge into the rather large umbilicus. The 

 surface, as preserved in a mould of the exterior in the stone, is 

 marked on the upper side of the volution by ten or twelve mode- 

 rately strong revolving ridges, which are smaller and more closely 

 arranged towards the suture, where there is a slightly depressed or 

 flattened space. These are crossed by numerous less strong, closely 

 arranged transverse strige, which bend backwards from the suture, 

 and have a strong retral curve on the narrow concave band of 

 the periphery. Surface characters of, under side undetermined. 



This species is so entirely distinct in its form and surface characters 

 from any other species yet known in our Silurian rocks, that it can be 

 readily distinguished. It is more nearly allied with forms such as P. 

 hicina of the Upper Helderberg and Hamilton groups of New York. 



Formation and Locality. — In limestone of the age of the Niagara group 

 at Racine, Wisconsin and Bridgeport, Illinois. 



