PARTULA. 119 



ward from the high interior to the coast. The general trend of each of 

 these valleys is perpendicular to the coast-line situated at its base, and thus 

 it comes about that adjacent valleys are approximately parallel each to each, 

 and separated from each other by a sharp-edged ridge. Many of the valleys 

 are very broad at their bases near the sea-coast, but they all become narrow 

 gorges in the interior of tlie Island. The sides of the valleys are steep, 

 often 35°-60", and in many places precipitous. The copious rain-fall on the 

 upper slopes is sufficient to maintain a stream in almost every valley. The 

 valley-bottoms are thus well watered and support a luxuriant growth of 

 forest trees, such as the Tahitian chestnut (Inocarpus edulis), the " Purau " 

 (Paritium tiliaceum), and others. The wild plantain, or "Fei" (Mussa trog- 

 lodytarum Linn.), Turmeric (Curcuma), and Dracaena grow in abundance in 

 the moist earth of the valley-bottom, while the Caladium (Colocasia) clusters 

 along the banks of the stream, and the ground under the trees is covered 

 witli a carpet of moss and ferns. This vegetation extends for a considerable 

 distance up the sloping sides of the valley, and high np among the moun- 

 tains the wild plantain and Dracaena often form a continuous forest crossing 

 the ridges from valley to valley. Lower down, however, the ridges between 

 the valleys are covered by a tangled mass of reeds and guava bushes, or are 

 wholly barren of vegetation for long areas ; and probably constitute a bar- 

 rier over which it is impossible for snails to pass from valley to valley. 

 Indeed the physical features of the Island, in so far as the nature of the 

 valleys is concerned, are strikingly similar to the conditions found on the 

 Island of Oahu of the Hawaiian Islands ; and in Tahiti, as in Oahu, we 

 find that the snails of each valley are possessed of marked and peculiar 

 characters which distinguish them more or less from the snails of adjacent 

 valleys. 



It is evident that we have in Tahiti an opportunity to study the range of 

 variability of the several, more or less isolated colonies of snails living in the 

 different valleys, and to determine the relationships which may exist between 

 the various forms. 



Snails were collected in the following six valleys : Pirae, Hamuta, Faii- 

 taua, Tipaerui, Maruapoo, and Vaihiria. Pirae, Hamuta, Fautaua, and Tipae- 

 rui valleys are approximately parallel each to each, and are situated on 



