or 
INTRODUCTION. 
such as Nassa sequijorensis, A. Adams, would form a part, and a 
very important part, in this line of descent. One of the series of 
shells in which the most gradual and almost imperceptible grades 
of variation takes place is that commencing with the Nassa turrita, 
A. Adams, and ending with a shell very little larger than Rissoa 
costata, viz., the Nassa costulata, found fossil in the Miocene of 
Bordeaux. This series includes the Nassa limata, Chemnitz ; pris- 
matica, Broechi; lucida, Marrat ; proxima, striata, and versicolor, 
C. B. Adams; denticulata and rufocincta, A. Adams ; crebristriata, 
Carpenter ; annellifera, Reeve ; ambigua, Montague, etc. 
In a long series of forms, commencing with shells repre- 
senting the largest specimens in the genus, these can be traced with 
unerring certainty into others forming the smallest examples known 
to exist ; again, the broadest varieties can just as easily be connected 
with others that are the narrowest examples in the group; and 
every grade of difference throughout the long lines of progressive 
variation is distinctly seen. 
In a case of the shells having smooth forms, such as N. glans, 
Linn., the varieties may not be all smooth specimens, but they may 
vary into costate and even cancellated examples. Again, instead of 
there being a uniform rate of variation of thickness observable, one 
set will be almost transparent or thin and hyaline, and another 
thick and quite opaque. The sculpturing is in many cases con- 
fined to the upper whorls, but we find shells in which the pattern 
is commenced on the top gradually spreading in successive develop- 
ment until it covers the whole shell; in one case it may form 
smooth unsculptured ribs, or in another it may diverge into any of 
the forms of sculpture we meet with in other groups of shells. 
The tip of the spire may be of the same colour as the remaining 
portion of the shell, or it may vary into almost every shade of 
pink, rufous, brown, purple, or almost black. 
The shells used as starting points are not intended as distinct 
forms, nor are they anything more than varieties chosen for the 
purpose of illustrating the subject of variation through a 
number of its ramifications. All the divisions marked with a dash 
will be found to assimilate with each other in some of the varieties 
