PARTULA HYALINA. 45 
The shell of hyalina has been described as practically invariable, not only 
throughout Tahiti but also in the whole field of its occurrence, which includes 
certain islands of the distant Cook and Austral Groups. Garrett calls particular 
attention to this uniformity as a remarkable feature of the distribution of this species, 
pointing to the diverse geological features of the elevated coral-limestone islands 
of the Cook Group as contrasted with the volcanic character of Tahiti. Yet, 
somewhat inconsistently, he also speaks of minor variations among /yalina such as 
“would be due to differences in food and station,” thus attributing a high causal 
value to environmental circumstances. 
The material for the present study of variation in the measured characters of 
the shell consists of 463 adult specimens, out of the entire number of 589. On 
account of their fragile nature, many shells are broken at the time of collection or 
transportation, so that all are not available for statistical treatment. The perfect 
shells have been examined and compared, valley by valley and quadrant by quad- 
rant, on the basis of all of the seven characters specified in the introductory section. 
The embryonic contents of nearly the entire series have been taken into account 
also. But it must be plainly recognized that the several colonies of this species do 
not always comprise enough individuals to make a statistical analysis significant; 
were there as many specimens as in the case of P. otaheitana, we might compare the 
colonies valley by valley with entire confidence in the figures. In only a few cases 
do the numbers at hand rise above a score. Hence it has seemed best to proceed 
from a general to a more particular statistical description of the shells of this species, 
taking into account in the latter case only a few characteristic valleys situated in 
the several ecological and geographical divisions of Tahiti. 
TaBie 12.—Partula hyalina. Tahiti. General statistical description of 463 adult shells. 
Character. Range. Mode. Mean value. Standard deviation. 
Shell: 
ILA, WANG Go o0000 12.6 to17.7 | 15.35 | 15.3098= .0245 0.7802 .0171 
WAG, TN, Goog0000 Uoll 10.1 8.70 8.7415 .0130 0.4156+ .0091 
Proportions, p. ct....| 51.5 63.5 | 57.50 | 57.0227+ .0650 2.0687 + .0453 
Aperture: 
Length, mm........ 7.4 to 10.2 8.80 | 8.9024+ .0150 0.4782 .0105 
Width, mm......... 5.4 7.6 6.40 6.5028+ .0121 0.3852+ .0084 
Proportions, p. ct....| 65.5 82.5 | 72.50 | 72.9514 .0723 2.3034 .0504 
Length aperture + 
length shell, propor- 
ClONS DACULA ere 52.5 66.5 | 57.50 | 58.0378 .0518 
— 
.6493 = .0361 
Taking all of the available material of this species as it exists in Tahiti, the 
data relating to the seven essential characters (table 12) provide an accurate quan- 
titative description of the shell formed by this animal. Moreover, figures 1 to 10, 
plate 20, show certain characteristic examples of the species. Specifically the shell of 
figure 1 is the shortest specimen, taken in Tautira Valley; in spite of its small size 
the animal was nevertheless an adult, for its brood-pouch contained several young. 
The next shell, figure 2, is almost equally abbreviated; it is the broadest of the 
