14 INTRODUCTION. 
all further remarks could only be directed towards the elucidation 
of elaborate structure, having a tendency to corroborate the broad 
facts laid down, and more fully to illustrate the minor particulars 
in many of the changes observed. 
Some of the abnormal specimens of Nassa nodifera, Powis, 
have the ribs most irregular in width, and apparently without any 
kind of order with regard to their arrangement. Instead of starting 
with a narrow rib and proceeding in a regular line to the broadest, 
or vice versa, the narrowest frequently occurs either beside the 
broadest or between two broad ones. ‘These different breadths of 
rib in the same shell may be accounted for in the following way, 
viz., by the uniting of two or more of the tubercles at the sutures, 
and in confirmation of this we have the partial union of two in one 
case and three in another in a specimen of N. levigata, Marr., on 
which the ribs have just commenced forming, and have not oblite- 
rated the grooves between the approximate tubercles. Another 
departure from the ordinary rule of a single rib to a single tubercle 
occurs, and that rather frequently, in specimens of Nassa arcularia, 
Linn. It is the uniting of two or three ribs in a single tubercle, 
rendering the direction of the united ribs almost like the radii of a 
circle, This is variation in a directly opposite way to that of the 
former case, both of which occur among the abnormal forms. 
The nucleolar whorls forming the last volutions of the spire 
are said to afford good specific characters ; but a careful comparison 
with both hand-glass and microscope has failed to produce any 
satisfactory results with regard to their constancy. These characters 
only appear distinct when we deal with distinct and selected varie- 
ties; but when the intermediate forms come to be examined in 
detail the-characters are no longer satisfactory. 
The evidence brought forward in the following pages is not 
intended to be in a consecutive form, but shows more particularly 
the extraordinary amount of variation to be found in individual 
shells, and how they pass by almost imperceptible degrees from one 
to the other. Weare aware of the utter impossibility of accom- 
plishing the task of uniting the whole of the shells collected as 
varieties and showing the exact progress each line has made 
