rhcves 
32 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
technical staff of the American Museum of Natural History, under the close super- 
vision of the author; plates 9 to 15 show the various aspects of this model from the 
zenith and from the cardinal points of the compass, together with outline keys. 
In addition, the photographs of plates 5, 6, 7, 8, 15, 16a, 17, 18, and 19, illustrate 
the topographical and other features of representative valleys. The table and 
plates are sufficiently comprehensive to define correctly the essential facts that are 
related in the following systematic description. 
The two extinct volcanic cones of which Tahiti is composed are undoubtedly 
of ancient origin, as their rugged centers and slopes indicate. These cones have 
been weathered so as to be furrowed by radiating valleys of greater or less width 
with steep or gradually sloping sides. ‘The firmer rocks that have resisted weather- 
ing and the masses of greater original height form the buttresses and intervening 
ridges, which rise to nearly perpendicular pinnacles in the central parts of both 
divisions of the island. The softer materials have been washed down by the almost 
daily rains to form a coastal plain or shelf that may be but a few yards in width or 
that may extend back for a mile or more in the radii of the larger valleys. From 
a few yards to a half mile outside of the shore-line runs a living coral reef which is 
interrupted only here and there, so that nearly everywhere the coastal plain is 
protected from the wash of the powerful waves; fringing reefs also occur at several 
points on the coast. ; 
In Tahiti nui the main peak reaches an altitude of approximately 7,500 feet, 
while other peaks near the center range from about 5,000 to 2,500 feet. The central 
portion of this division of Vahiti, within a circle drawn about 6 miles back from the 
coast line and concentric with it, is extremely rough, for in this area were the prin- 
cipal and secondary vents of the ancient volcanoes, now partly filled, but forming 
deep hollows between and among the steep-sided and sharp-pointed peaks. By 
weathering, however, the sides of these craters have been cut so as to give outlet 
to the waters that might otherwise have collected to form lakes, although one 
exception occurs, namely, Lake Vaihiria, 2.5 miles due southwest from the geo- 
metrical geographical center of the island, at an altitude of over1,400 feet. ‘These 
great outlets that appear from a distance offshore as huge clefts in the sides of the 
cone form valleys of a primary order that communicate centrally, though at relatively 
high altitudes and in nodirect manner. ‘There is no central plateau, and steep ridges 
of considerable height must be crossed in passing from the head of one primary 
valley to another. 
For administrative purposes, the island is divided into 19 districts—14 in Tahiti 
nui and 5 in Tahiti iti or Taiarapu. The boundaries of these sections are marked 
by prominent topographical features in the way of high peaks and ridges forming 
the borders of primary and secondary craters, as well as major divides that radiate 
outwards from the center. For instance, Papenoo is bounded on the west by the 
lofty border of Tetiairi Valley near the coast and of Papenoo Valley itself further 
inland; the border-line culminates in Orohena (7,321 feet), and follows the contour 
of the great crater southward and eastward to the peaks of Tetufera (about 6,000 
