456 Fossils of the Chazy Limestone. 



The specimens are from three to fourteen lines in width, and 

 the length varies from a little more than one-half to four-fifths of 

 the whole width. The variation in the proportional length and 

 width is partly owing to differences in the thickness of the shell. 

 Some have the shell so greatly thickened below that one or two 

 lines is added to the length thereby. It is well known that many 

 of the mollusca of the existing seas are subject to great variations 

 in the thickness of the shell. 



This species closely resembles the Raphistoma staminea (Hall) ; 

 but as Professor Hall says in his description of the genus (Palaeon- 

 tology of New York, vol. 1, page 28) that the umbilicus is 

 " moderately large," I infer that the three species described by 

 him must be umbilicatecl. In page 29, he states that "the striae 

 (on the surface of R. staminea) bend abruptly forwards, and,, 

 curving gently round, pass into the umbilicus " ; from which 

 expression no other conclusion can be drawn than that R. stami- 

 nea does possess an umbilicus. Our species is not umbilicated; 

 and, therefore, I believe it to be distinct from R. staminea. 



Locality and Formation. — Island of Montreal. Chazy Lime- 

 stone. 



Collectors — Sir W. E. Logan, J. .Richardson. 



Pleurotomaria Crevieri, N. s. 



Figs. 33, 34, 35. 



Fig. 33. — Side view of Pleurotomaria Crevieri* 

 34. — View of spie. 

 35. — Side view of a different specimen. 



Description. — Shell small ; whorls four ; spire nearly fiat ; base 

 sub-hemispherical ; no umbilicus. 



On the upper surface the first two whorls form a low, rounded 

 elevation in the centre, rising a little above the outer margin ; 

 the others are gently concave, and a little depressed below the 

 margin of those preceding. The base is sub-hemispherical or 

 depressed conical, the length a little more than half the width of 

 the spire. The surface is marked with fine striae of unequal size, 

 curved as in all the other species of this group. The outer margin 

 is acutely rounded, not bevelled as it is in specimens of P. calyXy 

 of the same size. Width of spire,, five or six lines. 



