ECOLOGICAL CHARACTERISTICS OF MARIANA ISLANDS. 5 
single island possess different varieties of a widespread species in correlation with 
their separation by barriers to the free inter-communication of their organisms. 
Like all general statements, however, qualification is necessary in exceptional 
instances, as for example in the case of Partula hyalina, which as one and the same 
species inhabits Tahiti as well as certain members of the Cook and Austral Groups. 
The Mariana Islands are more than 4,000 miles from the present headquarters 
of the genus in the Society Islands. They lie in the extreme northwestern part of 
the whole range (cf. plate 1, opposite p. 13, volume on Tahiti); together with the 
long complex chain of the Caroline Islands, the Bonins, and a few scattered ele- 
ments, they constitute the subregion of Oceania called Micronesia. In Parts 79 
and 80 of Pilsbry’s Manual of Conchology, 10 species are attributed to Micronesia 
as a whole, as follows: Pelew Islands, 3; Caroline Islands, 4; Mariana Islands, 3. 
The present investigation increases the last number to 4; and these are particularly 
interesting on account of their remoteness from the generic headquarters in Poly- 
nesia, and also on account of their contrasts in certain ecological respects with the 
species of Tahiti and its neighbors. The second subregion of Melanesia lies to 
the south of Micronesia, with 37 species on record from the Fiji, New Hebrides, 
Santa Cruz, and other contiguous groups of islands. The third and largest sub- 
region, Polynesia proper, occupies the southeastern part of the whole area of 
Oceania, and from this division more than 60 species have been secured by natural- 
ists. The prominence of the Society Islands in this subordinate area is attested 
by the fact that the 6 largest islands of the group are the homes of as many as 44 
of the Polynesian species. In brief, then, the Mariana Islands bear few species of 
the genus, and they lie quite at the extreme northwestern limit of the whole oceanic 
area in which Partula occurs. 
GEOGRAPHY, GEOLOGY, AND PHYSIOGRAPHY. 
The long chain of islands which constitute the Mariana or Ladrone Group 
comprises 15 elements of smaller and larger size, together with a few scattered reefs 
and banks. They are extended along the line of an almost perfect upright bow with 
its convexity toward the east. Guam is the southernmost and largest member of 
the group (text-fig. 1), and its own southern point is located approximately at 
latitude 13° 14’ north, longitude 144° 40’ east. The northern end of the arc is 
marked by the small volcanic cone of Uracas, at latitude 20° 30’ north, longitude 
144° 55’ east. The point of greatest convexity is occupied by the island of Medi- 
nilla, which is exactly at the junction of the sixteenth parallel north with the 
meridian of 146° east. These figures indicate the slight degree of curvature of the 
entire submarine range, 420 miles in length, whose peaks constitute the several 
islands of the group. 
A line running almost due north-northeast from Guam (text-fig. 1) leads first to 
Rota, 30 miles distant, and then to the small island of Agiguan, 45 miles beyond Rota. 
A short interval of only 5 miles separates Agiguan from Tinian, 10 miles in length. 
Saipan is next in order, only a little over 2 miles from Tinian; it is next in size to 
Guam, being about 10 miles in length and slightly less than 5 miles in greatest width. 
