138 TRANSACTIONS OF THE [MAR. 9, 
insensible gradations, and, placing the most extreme forms at 
the termini of the branches, such a tree as I have represented 
was the result. But, having once constructed such a tree, no 
other arrangement is possibie. We cannot, for example, pass 
from 6 to 6’’ except in the manner indicated, 7. e., by passing 
down one stem and up the other. Indeed, we can give to almost 
every shell which was obtained a place corresponding to one of 
the numbers of the figure. A very few slightly aberrant forms 
were found, to be sure, but I thought it best not to complicate 
the tree by introducing them. Three such are represented in 
the right-hand lower corner of the figure. 
It will, doubtless, be asked why I have assumed 0 to be the 
typical form and placed it at the apex. This I did for two 
reasons: Firstly, this variety preponderates greatly in number. 
Out of 538 specimens which were obtained, 318 belonged to the 
three central numbers of the series (0, 1 and 1’) while the re- 
maining 220 (with a very few exceptions) belonged to the other 
fourteen numbers. Secondly, the genus seems typically to pos- 
sess three bands. Of the 22 species, or thereabouts, obtained 
by the expedition, thirteen exhibit bands either constantly or 
occasionally, and of these, nine either constantly or occasionally 
exhibit three bands. Moreover, we have reason to believe that 
the others have been modified through the fusion or loss of 
some or all of them. 
Examining our tree, we find that the central member (0) pre- 
sents upon each whorl three bands of approximately equal width 
and equal depth of color, all being of a dark black-walnut shade. 
Passing up the right hand branch of the series, we find that the 
upper band begins to narrow and to pale until it is completely 
lost. This happens also with the lowest band, only this one 
disappears a little later than the upper one. The central band 
persists to the last undiminished. On the left-hand branch, 
we observe a quite different transformation. The upper and 
lower bands begin to lose their sharp outlines, becoming blurred 
along their edges. This process of spreading continues until 
the color has diffused itself toward the central band, filling in 
the white interspaces. The effect is the same as if the pigment 
had: been fresh paint and had been first laid on in streaks and 
then spread out witha brush. From about the position of 3/ 
two branches now lead off, presenting opposite tendencies. Both 
agree in continuing this diffusion of color and obliteration of 
the bands, but whereas on the left hand side, the color deepens 
and the entire mass is now nearly as dark as were the original 
markings ; on.the right hand the color pales and the result is a 
mustard tint. Inthe dark form,again, the central band persists 
