230 ARTHUR WILLEY. 
ture,! and it is important to notice that they can be arranged 
in three groups as follows : 
1. Single variations, in which the variation affects only 
a single character, such as the umbilical shoulder or the um- 
bilical aperture. 
2%. Collective variations, in which a whole group of cha- 
racters are affected, as in the variety Moretoni. 
3. Incomplete variations, as in the two examples of my 
variety marginalis, where the umbilical shoulder was con- 
fined to one side of the shell. 
Finally, it may be pointed out that the species and varieties 
of the recent Nautilus seem to fall naturally into the following 
scheme, which, however, does not aim at suggesting any 
particular direction of evolution. 
N, macromphalus. N. umbilicatus. 
N. umbilicatus, v. secrobiculatus. 
N. pompilius, v. Moretoni. 
N. pompilius, v. stenomphalus. 
N. pompilius, v. perforatus. 
N. pompilius, v. marginalis. 
Ve 
fr 
N. pompilius. 
University oF SYDNEY ; 
May 18th, 1896. 
1 Bareson, W., ‘Materials for the Study of Variation,’ Macmillan, 1894. 
