31 



This species is closely allied to P. Helena of the Hudson River formation 

 but differs in being mucb smaller and in having the upper side of each 

 whorl gently convex on the inner side and concave on the outer side 

 whereas in P. Helena the whole of the upper side of the whorl is .gently 

 concave. 



It differs from P. caldfera and P. rotuloides in having the umbilicus 

 closed. 



Locality and Formation. — Campement d'Ours, near the Island of St. 

 Joseph, Lake Huron. Black River limestone. 

 Collectors. — A. Murray, R. Bell. 



Plburotomakia Arachne. (N. sp.) 



Fig. 32. 



Description. — Shell small, turbinate ; whorls three or four, ventricose 

 spirally carinated ; apical angle about 75°. There is a broad spiral band 

 situated with its upper edge about the middle of the whorl ; above this and 

 only separated from it by a sharply elevated thread-like carina there is a 

 second very narrow but well defined concave band, above which the whorl 

 is gently concave for about two thirds the distance to the suture ; and in 

 the remaining third to the suture there is a spiral row of small elevated 

 imperfect varices which give to this part of the shell a peculiar nodulose 

 appearance. The suture is deeply canaliculate. Under side of body whorl 

 moderately ventricose. There is a small umbilicus. Aperture ov.al some- 

 what effuse below. 



Surface with elevated sharp-edged lines of growth, distinctly visible to 

 the naked eye, from eight to ten ki the width of one line. On approach- 

 ing the suture every two or three of these unite into one in order to form 

 the short varices which are situated near the suture. They also unite in 

 the same manner on the lower part of the last whorl as they approach the 

 umbilicus. 



Length from 5 to 10 lines. The most perfect specimen examined is 5 

 lines in length and 4 lines in width at last whorl including the width of the 

 aperture. The latter is 2J lines in height and 2 lines wide. The larger 

 spiral band is 1 line wide at the aperture and the smaller about | of a line. 

 There are three narrow sharply elevated cariase, one bordering the lower 

 side of the larger band ; another separating the two bands and a third 

 bordering the small band on the upper side. 



Locality and Formation. — Pointe Clahre ; Paquette Rapids ; and 

 Murray Bay. Black River limestone. 



Collectors.— ^vc W. E. Logan, Pr. J. W. Dawson, E. Billmgs. 



