Raymond : Gastropoda of the Chazy Formation. 177 



When the shell is destroyed, a cast of the interior exhibits an um- 

 bilicus one eighth the whole width of the spire. 



Some of the specimens are two and a half inches wide. 



Pleurotomaria immatura Billings. 



Spire nearly flat. In specimens an inch and a half wide the apex is 

 elevated four or five lines above the margin. Owing to the depression 

 of the outer whorl, the spire is semi-turreted. 



Surface marked by rather fine backward-curving striae. 



Pleurotomaria calyx Billings. 



Spire nearly flat, acute, or sharply rounded on the outer edge. No 

 umbilicus. Apex slightly higher than the margin. This species 

 closely resembles the Raphistoma stamineum Hall, but is not umbicu- 

 lated, and therefore Billings believed it to be distinct from Raphistoma 

 stamineum. 



Pleurotomaria crevieri Billings. 



Spire nearly flat. No umbilicus. The anterior margin is acutely 

 rounded, not beveled, as in specimens of Pleurotomaria calyx of the 

 same size. Surface marked by fine striae of equal size as in all the 

 other species of the same group. 



Pleurotomaria pauper Billings. 

 Shell small, flat above, ventricose below and obtusely angulated at the 

 edge of the umbilicus. Surface markings unknown. This shell is 

 about the size and shape of Pleurotomaria crevieri, but with a per- 

 fectly flat spire and an umbilicus. 



Observations on the previous species. 

 A large number of well-preserved specimens from Chazy, New York, 

 which appear to represent one species, show a great range of variation. 

 These variations are : first, in the relative height and width of the 

 shells ; second, when the shell substance is removed all specimens 

 show a wide umbilicus, but when the test is complete the umbilicus is 

 nearly or quite concealed ; third, the spire may vary from almost per- 

 fectly flat to one which rises several millimeters above the level of the 

 outer whorl ; fourth, the surface markings of young shells consist of 

 fine impressed lines, while adults of the same species have coarse 



