PARTULA, RAIATEA AND TAHAA. 213 



the angle on the front differentiate P. erhelii from otherwise 

 similar known Moorean forms. M. Morelet states that it 

 was brought from Tahiti by M. Erhel, a young surgeon of 

 the marine service who died in Senegal. 



3. Species of Raiatea and Tahaa. 



The Partulse of these two islands are so intimately related 

 that they must be treated together. The separation of the 

 islands by a shallow lagoon about two miles wide, has evi- 

 dently been a quite recent event. Both islands are enclosed 

 in the same encircling reef. 



The extrinsic relations of the Partulae are with Tahitian 

 forms. With the exceptions of P. turgida (no. 8) and P. 

 attenuata (no. 17) already described, all Raiatean species 

 may credibly be assumed to have descended from a single 

 ancestral stock which was also the common parent of the 

 otaheitana group. Subsequent events were as follows. (1) 

 Three chief branches of this stock arose : the faba group, the 

 dentifera group and the hebe group. (2) Each of these 

 stocks spread over the available area, and (3) by local differ- 

 entiation gave rise to numerous races, which are still doubt- 

 less spreading and becoming further modified. This tertiary 

 geographic radiation and racial differentiation was also ac- 

 companied by an adaptive radiation whereby several arboreal 

 forms gave rise to terrestrial species. As the result of these 

 events we find most available stations occupied by two to four 

 species, each representing one of the three stocks derived 

 from differentiation (1), and from the adaptation to terres- 

 trial conditions. 



The comparative simplicity of the distributional conditions 

 indicates that the evolution cycle outlined above has not 

 been of great duration. By analogy with known cases, dif- 

 ferentiation (1) probably took place in Pliocene time. 



Most Partulag known to be terrestrial are from Raiatea and 

 Tahaa. The terrestrial habit is no doubt a readaptation of 

 arboreal ancestors. The coloration approximates to the dull 

 tints of ground snails. 



