PARTULA GIBBA, GUAM. 71 
unicolor agrees with other classes and gives no reasons for assorting the shells into 
two subordinate groups with narrower and wider white borders respectively. 
bicolor.—The second class is called bicolor because its members display the 
coloration regarded by Pilsbry as typical for the var. bicolor of Pease; no confusion 
need arise from the employment of the identical word in different senses when it is 
explicitly stated that it is here used to designate a color-class. 
This group stands close to the preceding, with which it agrees in the general 
ground-color and in its interior tints. The chief difference is that bicolor possesses 
a revolving band of deeper color on the uppermost whorls (figs. 15 to 24, plate 12). 
This band varies from yellow to orange-brown; its upper border is vague, but the 
sutural boundary offers a sharper contrast with the paler portion of the adjacent 
whorl below. The lip is stained with light yellow, yellowish brown, or orange- 
brown, although in some instances it is white as in unicolor (table 31 of next sec- 
tion). There seems to be some correlation between the distinctive band and the 
lip as regards the depth of color (compare figs. 17 and 18, 22 and 23, plate 12), but 
this is not absolute. 
The Tarague bicolor are straw-yellowish corneous, like their unicolor and 
mitella associates, and the few other specimens taken in the Northeast Region at 
Asados and Santa Rosa agree with them. In all other places bicolor is more pallid. 
Decortication occurs in a considerable number of individuals, always later in life, 
for the adolescent examples (fig. 20, plate 12) never display its effects. A Tarague 
shell so altered (for example the shell of figure 16, plate 12) may come to resemble 
the pallid kinds of other regions, but is of course really different in nature. In the 
early stages, the bicolor shells do not show the revolving band with clearness; this 
feature becomes intensified as maturity approaches, and consequently it is not 
possible to assort the unicolor and bicolor adolescents with certainty when the two 
kinds occur in the same association, as at Tarague. 
The collections from 15 localities comprise bicolor shells, but the areas in ques- 
tion are by no means in geographical sequence (consult table 29). Far to the 
extreme northwest the type occurs in considerable absolute and relative numbers, 
while it constitutes the sole representation of its species in the Southwest Region 
at the other end of Guam, where, it will be recalled, the shells are extraordinarily 
large (fig. 24, plate 12). In the intervening territory it is found only locally and in 
sparse numbers for the most part, judging as we must from the actual collections. 
Certainly the figures are significant in the comparison of the Fonte-Asan territory 
with the two Macajna areas. A final point is the noteworthy fact that bicolor is 
lacking in the Lolo series, in which unicolor is so largely represented. 
mitella—By far the largest number of gibba in Guam display the “hooded” 
coloration which is indicated by the chosen designation, mitella (figs. 25 to 44, plate 
12). This is Férussac’s original type, and Pilsbry also recognizes its primacy; 
apparently Férussac’s figures were made from a decorticated specimen with the 
typical yellowish tint of the body greatly altered. 
Again we begin with the Tarague shells in which the general ground-color is 
corneous with a considerable amount of yellow, as in the associated unicolor and 
