LINES OF DESCENT. 87 
persons have seen a shell of the Nassa incrassata, Miiller, at least 
twice as broad, and with a much more expanded lip, than the large 
variety figured in Reeve’s “Conchologia Iconica,” notwithstanding 
there are such shells and many other curious forms equally unknown 
to conchologists. It is such varieties as the specimens just 
mentioned that have enabled me to unite so many of the spurious 
species. i 
At least two hundred examples of these shells are in the trays 
before me, and the diverging forms appear to be very numerous 
among them. 
Some of the names given in this list are only synonymes, 
others are known to be varieties, and have been generally recogiised 
as such by conchologists. ‘There is not a group, nor scarcely a shell 
in any of the groups, that one or other of the varieties of Nassa in- 
crassata, Miiller, does not approach rather closely, and _ the 
intermediate forms between it and other named shells are very 
numerous. A recent shell is so like the fossil N. asperula, Brocchi, 
that it is a difficult matter to determine in what respect they differ 
the one from the other. 
The most appropriate name for this shell is that applied to it 
by Philippi, viz., Nassa variabilis, for a more variable shell is not to 
be found either in this or any other genus in the whole category of 
shells. 
We have given this example as an illustration of one of the 
lines of descent, but not necessarily terminated at either end, 
neither do we profess to give the varieties in their consecutive order. 
There are two other groups closely allied to this, and several of 
the shells belonging to each pass from one to the other by im- 
perceptible gradations. 
One is composed of shells covered with granules or papille, 
and includes the N. gemmulata, Lam., and its variety verrucosa, 
A. Ad., N. granulata, Marr., &c. The shells in the other group 
are more or less ribbed and cross-grooved ; included in this 
series is the N. prismatica, Brocchi, N. versicolor, C. B. Ad., 
N. striata, Reeve, &e. 
