6 INTRODUCTION. 
occurring in each section. The N. nodifera, Powis, is a costate 
variety, intermediate in its character between the smooth shells of 
N. trifasciata, Gmel., and the cancellated forms of N. marginulata, 
Lam., and similar remarks will apply to all the shells forming the 
starting points. 
The arrangement of these shells is purely and simply as it 
exists in Nature, and only requires the student to examine it without 
paying any attention to the specific distinctions propounded in the 
works of the most learned conchological authors, to be convinced of 
its correctness—Nassa 18 ONE SHELL IN AN ENDLESS VARIETY OF 
Forms. 
In my Paper on the “ Variation of Sculpture,” some of the 
modes in which these shells vary have been pointed out ; others, still 
more complex, remain to be explained. Again, starting with the 
smooth forms, the first and possibly the most abundant variation 
is that in which delicate longitudinal lines appear all over the 
shell; at first they are irregular and interrupted, but at length they 
appear with tolerable regularity ; then the cross-grooves make their 
appearance, beginning with a few and increasing in number until 
we have a finely cancellated shell; this is easily observed in 
varieties of N. planicostata, A. Ad., and N. labida, Reeve. 
As the lines increase in size, and the cross-grooves in depth, 
the external pattern becomes larger, until it is of a very coarse 
kind,—this is only one of the simple forms ; another, and one that 
is very common, is the different sizes of the ribs, traversed by 
cross-grooves, forming somewhat square spaces by their intersection, 
—these have rounded surfaces, derived from the rounding of the 
ribs (N. reticulata, Linn., is an instance). A curious instance of 
the square spaces formed by these intersecting lines being placed at 
nearly equal distances from each other is seen in specimens of the 
N. nodulosa, Marr. Smaller and more numerous squares may be 
seen on the N. cremata, Reeve (not Hinds!), N. ravida, A. Ad., ete. 
In other shells we find, not square spaces, but elongated ones 
passing into parallelograms of different sizes, and sometimes placed 
at slight angles to each other. Instances of this sculpture are 
found in the N. stigmaria, A. Ad., etc. The nodules are 
