102 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
yellowish corneous (figs. 1, 2, plate 14) both on the outside and within. Deepen- 
ing of the color leads to the full yellowish phase (figs. 3 to 8, plate 14), which is 
very rarely modified by the addition of brown (fig. 6, plate 14); here too the inner 
color corresponds closely with that of the outer surface. As in the paler shells, 
the lip is always pure shining white. Finally, in this class there are shells which are 
almost white (fig. 9, plate 14), usually as the result of decortication; but some 
adolescents are so pale as to suggest as early differentiation of this kind of shell 
(fig. 10, plate 14). From the census table it is apparent that this class varies 
greatly in its proportionate abundance, from 65 per cent at Sadog Tase to 1.33 
per cent at the neighboring station of Puntan Muchut, and to none at Torre and 
Garapan. Possibly it is a group whose members prefer the higher levels, for the 
Sadog Tase, Fanaganam, and Astaman series were taken in the bush of greater 
altitudes. Yet the seeming relation to barometric level may be only a secondary 
and unimportant feature. , 
The bicolor class is the dominant one in Saipan, where it preempts the position 
held-by mitella in Guam; it is not only the most abundant but it is also present 
in all of the representative collections, even though its numbers amount to less 
than 1 per cent as at Chalankiya. The Fanaganam, Torre, and Astaman series 
are made up largely of this class. Its distinctive features are the same as in Guam 
(figs.11 to 20, plate 14),namely, a revolving band of varying shades of yellowish brown 
on the whorls of the spire, and a lip that is tinged lightly or darkly, in some correla- 
tion with the depth of color of the rotating band. The ground-color is usually 
corneous as in unicolor (figs. 11 to 13, plate 14); from this the color deepens in other 
examples to yellowish corneous (figs. 14 to 16, plate 14), but it rarely attains the 
rich hue of the yellowest unicolor shells. Decortication results in the lightening of 
the general color (figs. 17 to 19, plate 14), and by virtue of this change the revolving 
bands stand out more sharply by contrast. The adolescent shells show very little 
of the band until they have attained a considerable size (fig. 20, plate 14). 
The mitella class presents no novel features as compared with that of Guam 
(figs. 21 to 30, plate 14). The general color grades from corneous to brown-corneous, 
the color of the spire is light orange at one extreme and red-brown at the other, 
while the lip is stained with yellow, yellow-brown, or orange-brown. When the 
shells are decorticated (figs. 28,29, plate 14) the contrast between the body-whorl 
and the spire is accentuated. The typical tints of the spire are even later in their 
development than in the same class of Guam, being evident in a very diluted degree 
only when the shell is about half-grown (fig. 30, plate 14). 
The order distinguished as mitella-rubra is illustrated by figures 31 to 37, 
plate 14. As in Guam it is characterized by the red suffusion upon the body-whorl, 
which in a sense is an extension of the deep color displayed by the spire, but the 
two reddened areas are not always directly continuous (fig. 34, plate 14). The ~ 
deepest-colored shell (fig. 35, plate 14) is a solitary example from Astaman, and 
it is unusual in the dark color of the interior wall as well as in the depth of the red 
color on the outer surface. Decortication sometimes occurs in this class (figs. 36, 
37, plate 14). As the distinguishing feature of this order is manifested only when 
