SUMMARY. 95 
tracing the descent of recent from fossil forms; an arduous task in 
which an artificial system is certain to mislead its adherents. The 
writer’s aim will be accomplished if the foregoing imperfect notes 
shall in anywise prove helpful to future investigators in illustrating 
the derivation of recent from extinct forms amongst the objects of 
his study, 
EXPLANATION OF THE DIAGRAM ON THE 
FOLLOWING PAGE. 
I have constructed the following diagram for the purpose of 
illustrating the lines of ascent, as projected from a given point. If 
a shell be taken from any part of the series and traced in detail, in 
the descending order, it is certain to terminate in one or other of 
the varieties of Nassa incrassata, Miill. There are numerous 
branches projecting from these main stems of the diagram, but the 
order in which they occur has not been sufficiently determined to 
enable me to map them correctly. In each of the longer projected 
lines a very large series of varietal forms occur, and when the shells 
are placed upon them there is very little difficulty in tracing the 
lines of affinity. All the known forms in this genus belong to one 
or other of these five lines, for many branches after leaving the stem 
bend and turn back into the same line, higher up or lower down, as 
the case may be. The varieties of N. marginulata, Lam., are so 
numerous that we find them uniting with varieties in every ascend- 
ing or descending line of affinity in the series; hence the line has 
been placed across the five lines in the diagram, showing the inter- 
mediate position they occupy. I exhibited the shells composing the 
diagram as a branch in the pedigree of the genus Nassa, illustrating 
the affinities of 40 named forms with Nassa hirta, before the mem- 
bers of the Liverpool Literary and Philosophical Society, October, 
1880, and found the inquiring portions of the audience were satisfied 
with the result obtained, 
