343 



Surface with fine sharp striae of unequal size, the smaller four or five 

 in one line, the larger twice the size of the smaller ; usually with shallow 

 undulations one or two lines wide conforming to the course of the strige ; 

 all curving backwards to the margin, which they reach at an acute angle. 

 In some the surface is reticulated with longitudinal spiral lines following 

 the whorls to the apex. Diameter from 18 to 30 lines. 



Fig. 329. 



The whorls vary in their width greatly. Fig. 329 represents a cast of 

 the spire of an individual with six whorls, and specimens have been col- 

 lected which have them still more slender. 



P. aperta (Salter) is the species most nearly allied to this, the trans- 

 verse section of the whorls being precisely the same except that in the 

 umbilicus they are not flat but rounded, and, besides, it is a smaller form 

 and never has more than four whorls. The large specimen figured in the 

 Geol. of Canada, p. 117, fig. 28, d, as a form of P. Laurentina, belongs 

 to this species. The true P. Laurentina has the umbilicus scarcely one- 

 fourth the whole width. (See also ante, p. 191, 229.) 



Locality and Formation. Counties of Leeds and Grenville, also Min- 

 gan Islands ; Calciferous formation. 



PLEUROTOMARIA ARABELLA. (N. sp.) 



Fig. 330. 



Description. Shell small, with four or five slender whorls ; spire de- 

 pressed convex, with the whorls turrreted above each other for about one- 

 fourth their depth. On the upper side there is a deep narrow groove close 

 to the margin following the whorls to the apex ; all between the grooves 

 and the suture rather strongly convex, the greatest elevation being at 



