PARTULA GIBBA, GUAM. 63 
Partula gibba is the dominant species in the western half of Guam, and in most 
of the territory of the South Central Region as well. It is entirely absent from 
Cabras Island, and it is infrequent on the Orote Peninsula, judging from the single 
collection taken thereon. The numbers diminish with the passage to the Southwest 
Region, and the Inarajan collection comprises none of this species. 
The absence of gibba from the lower levels of the Ylig and Pago River valleys 
is noteworthy, because radiolata was found in considerable numbers in both locali- 
ties, while on the adjacent uplands gibba flourishes in abundance. The low altitude 
by itself does not seem to be responsible for its absence from the river gorges in 
question, because the species occurs at equally low levels on the western side of the 
island, notably at the Presidio stations and in the Coast Central Region. And 
higher altitude alone can not be regarded as a favoring factor when the species is so 
rare on the upper collecting-grounds of the Southwest Region. 
Here, again, we can say only that the numbers are sometimes few and some- 
times many in territories that seem to possess identical ecological requisites for the 
occurrence of any Partule at all. Apparently innate vigor and a substantial 
reproductive rate determine the numerical abundance of the species in any locality 
with suitable vegetation and the vital degrees of shade and moisture. 
COLONIAL VARIATION IN QUANTITATIVE CHARACTERS. 
The actual collections are sufficiently rich in almost all the localities examined 
to give reliable statistical definitions of the standard characters of the shells of the 
different colonies. For the sake of completeness, even the smaller series has been 
statistically analyzed, and hence the tables of the present section comprise the 
figures for all of the perfect and measurable shells. 
The full quantitative description of the species as it was found in Guam is 
given in tables 25 to 28. The local series are individually defined and the regional 
populations are so combined as to afford descriptions of the inhabitants of the more 
inclusive areas, while finally the figures are recorded for the entire series of shells 
taken as a single group. The last-mentioned data constitute an accurate quanti- 
tative description of the species, and hence they amplify, if they need not replace, 
the scanty figures and the qualitative terms employed by earlier authors. It would 
be impossible to obtain a more reliable empirical definition of the species without 
making a more complete collection from a fuller series of localities. 
By way of comment, it is clear that the ample local series, such as those of 
Tarague, Lolo, and Aniguac, exercise a greater influence upon the general averages 
than the smaller collections, like those of Saucio, Lonfit, and the southwest stations. 
This greater weight would be justifiable if the species were actually more abundant 
in the first-named places, which really seems to be the case; for example, many 
hours of work in the Umatac territory resulted in very sparse collections of gibba, 
while an hour or two in the Macajna or Fonte localities yielded an abundance of 
specimens. Only in the cases where a shorter period of field-work accounted for the 
collection of a smaller series where the species was actually numerous would a real 
error of under-weighting occur. But no misconception regarding the nature and 
