88 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
Shells of the dextral coil are regarded as more typical, save in the case of the colony 
termed var. /@va. ‘The specimens which are viewed as typical in color are brown or 
otherwise dark-colored examples (see figs. 26 to 30, plate 22) devoid of brown bands, 
but marked by a white sutural area; they constitute what is really a color-class 
rather than a variety in the sense of a subspecies, and although some other term 
referring explicitly to their coloration might well be employed, yet on account of 
their earlier definition they will be treated in the present discussion as forming 
the color-class nodosa, orthographically distinguished from a taxonomic variety. 
All of the banded forms are assigned by earlier authors to var. trilineata, although 
but two bands or only one may be present (see figs. 11 to 21, plate 22); but this group 
also is only a color-class and not a subspecies in the proper sense, wherefore the 
term is to be written distinctively, as trilineata. Light flesh-colored shells without 
the bands are, by Pilsbry, properly distinguished as “color-form pallidior’” (see 
figs. 9 and Io, plate 22). A special name has not been given to the color-type with a 
wide revolving zone, although it is as much entitled to independent status as any of 
the foregoing (see figs. 22 to 25, plate 22); it is herein called the color-form concrescens, 
because the broad zone is formed by the merging of two bands that are separate 
in trilineata. 
The four color-classes are intimately related and without question belong to 
one stock which has undergone a considerable amount of differentiation in respect 
to color and color-pattern; nevertheless, they are not physiologically distinct and 
independent to the degree that is necessary for their designation as taxonomic 
varieties. In the course of the following detailed analysis of the several valley 
colonies, differential characteristics of the nodosa colonies will be demonstrated, 
which belong to a category quite independent of the color classification. All four 
divisions established on the latter basis may or may not be represented in a colony 
which, as a whole, differs from another in certain other respects. Whether or not 
the color differentiation is more basic than the geographical differentiation does not 
need to be settled here. In the present research, attention is directed primarily to 
differences exhibited by isolated colonial groups of one and the same species; and 
if the terminology is to be made an aid to the main purpose, the regional classifica- 
tion may be made paramount, as in the case of P. clara, while the color classification 
may be, and hereafter is, to be used, if at all, for the practical purposes of precise 
description. ‘The biological points involved will receive due attention in the con- 
cluding part of this section. : 
THE GEOGRAPHICAL DISTRIBUTION OF PARTULA NODOSA. 
This species inhabits seven valleys lying in the western quadrant of Tahiti 
nui, which form a consecutive series beginning with Taapuna at the north and 
ending with Orofere at the south. The range of this species is therefore continuous 
and not interrupted, as are the ranges of P. clara and P. hyalina. In order to 
understand the significant features of the distribution in the case of P. nodosa a full 
description of the area of its occurrence is essential. 
