154 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
crater basin in the exact center of Tahiti nui, which is bounded by more or less lofty 
ridges, raised here and there to high mountain peaks. Outward from this depres- 
sion, and almost northward in general trend, the drainage area slopes gradually 
toward the coast and narrows so greatly as to be little wider than Fautaua and many 
valleys of the south, until it debouches upon the coastal plain. 
At its mouth Papenoo Valley is very broad and dry, and its limiting buttresses 
are also somewhat arid and therefore devoid of tall vegetation. About a mile inland 
the high sides draw together and the stream runs swifter and deeper, with many 
stony islands in its bed. On the level flood-plain, dry ferns, guava trees, and 
hibiscus form a characteristic plant association. Further on, where the slopes 
are higher and steeper, larger trees, turmeric, and succulent plants like Dracena 
are established, but no snails are to be found until a point nearly 2 miles from 
the coast is reached, at a barometric height of 75 feet. This is a low level for 
Partule, but the conditions of moisture and vegetation are such as to render the 
region habitable at a much lower point than in other large valleys, like Fautaua 
and Tipaerui. In the succeeding zone, further inland, the trees and shrubs grow 
still more thickly, and among them are the plantains, vines, and Pandanus that con- 
stitute the customary association for the harboring of Partule, which there occur 
in some abundance. The steep accessory gullies entering the main valley from the 
sides are especially favorable for the collection of snails. 
In conducting the present research it was impracticable to make the journey 
throughout the whole length of Papenoo Valley, and to cross over the boundary 
ridges into Fautaua or into Vaihiria. Collections of snails were personally obtained 
in 1906, 1908, and 1909; and as in the case of Pirai Valley, they were taken in 
different but nearly contiguous parts of the valley. In the first year, on account of 
the effects of the cyclone and the difficulty of penetration, only the lowermost 
periphery of the inhabited area was investigated up to a point about 3 miles from the 
coast; in 1908, snails were obtained from 3% to 4% miles up the valley, while in 1909 
the area of collection extended to about 54 miles inland. While it would be extremely 
interesting to investigate the wide circular basin in the center of Tahiti nui, the 
task would demand so great an expenditure of time and energy as to interfere 
seriously with the exploration of the peripheral parts of the island, and for this 
reason it was not attempted. It will be borne in mind, therefore, that the following 
description of the snails of Papenoo Valley includes only the positive results estab- 
lished for the drainage area which parallels Ahonu and Tuauru Valleys. 
The two color-classes are as well distinguished as heretofore. Among the 
yellow shells (plate 27, figs. 1 to 4) are some that are almost pure white (fig. 1) as the 
result of decortication. In the red class (plate 27, figs. 5 to 11) the tones vary some- 
what, according to the strength or weakness of the distinctive color; a pink colora- 
tion results from the wearing away of the epidermis (plate 27, fig. 8). The rich 
tints upon the lips and within the cavity of the last whorl, although they are not 
always developed, add much to the beauty of the shells of this division. A single 
diminutive example was found here (plate 27, fig. 11). 
