Xll FOSSIL ACHATINELLIIXE. 



so far specialized the dentition that it seems to me quite im- 

 possible that Achatinella could have been derived from any 

 Amastrine genus now living. 



The teeth of Physida may be a similar evolution from the 

 marginal teeth of some Basommatophorous stock. They have 

 a remarkable superficial resemblance to those of the Acha- 

 tinellidce. The Athoracophorida have also evolved teeth won- 

 derfully similar to those of Achatinella. This is evidently 

 a case of convergent evolution, as the relationship must be 

 remote. 



Notwithstanding its strong differential characters, the 

 family is a very compact one, with remarkably little struc- 

 tural variety for a group so prolific in species. Most genera 

 of Helices show much greater specific diversity in characters 

 of the soft parts than I have been able to find in the whole 

 family of Achatinellidcz. There has been no adaptive radia- 

 tion, and with the possible exception of Newcombia, there 

 are no aged or phylogerontic branches. 



This contrasts with the Amastrida, which have been adapted 

 to a variety of stations, humid, semiarid and arboreal, and in 

 which the shell varies from cylindric or turrited to discoidal, 

 with wide diversity in sculpture, color and solidity. 



It appears that the Achatinellidc? are a still youthful group 

 in the full flower of their evolution, probably derived from 

 some plain terrestrial form which was rejuvenated by the dis- 

 covery of a new station the leaves and branches of trees. 



Fossil Achatinellida known up to this time are all of 

 Holocene, or perhaps in part of Pleistocene age. They are 

 far less numerous than fossil Amastrida, probably because the 

 deposits wherein land shells are preserved lie mainly close 

 to the sea, while Achatinellidce are shells of the mountain 

 forests. The known fossil species follow. 



Achatinella phaeozona Gul. Kailua (p. 187). 



Achatinella casia littoralis P. & C. Kahuku (p. 266). 



Partulina montagui Pils. Manoa Valley (p. 66). 



Partulina dwighti occidentalis P. & C. Molokai (p. 361). 



Newcombia philippiana Pfr. Molokai (p. 356). 



Partulina confusa Sykes. Hawaii (p. 105). 



