Occasional Papers of the 3Iuseum of Zoology 115 



the absence of Cistulops and Tudora s. s. as points of differ- 

 ence. In this connection, it is at least interesting that the 

 direction of the ocean current is from Bonaire towards 

 Curasao, and from the latter to Aruba; it is considerably- 

 stronger between the two more western islands. 



Bonaire and Curasao agree in the absence of Thysanophora 

 and Oxystyla, and in the presence of the section Tudorata and 

 the species Succinea gyrata. 



These differences in molluscan fauna appear to be almost 

 purely geographical ; that is, they are coincident with degrees 

 of present or former isolation. The greatest ecological dif- 

 ferentiation is between the abundance of individuals and spe- 

 cies on the calcareous substrata and their paucity or absence 

 on the non-calcareous soil. Except for certain minor edaphic 

 phases, represented by the forms of Co-ion uva, the formation 

 on all of the islands is practically identical, as are also most of 

 the factors of the environment. In other words, the fonnation 

 content (the molluscan association or society) changes with 

 the area, although its general aspect remains the same. 



As an illustration of this, the habits of Tudora on Curasao 

 may be cited. In northern and southern Curasao, the species 

 of Tudora s. s. {T. fossor and T. pilshryi) are almost purely 

 terrestrial, while the species of Tudorata {T. rupis and T. 

 muskusi) are markedly subarboreal. In central Curasao, 

 Tudorata is absent, but T. (s. s.) megacheilos differs from its 

 closest relatives in a well-developed tendency to climb the 

 trees and brush, so that it practically occupies both the ground 

 and lower arboreal strata. Tudora fossor aruhana also shows 

 this tendency, although it is much less prominent. 



In this connection, it may be remarked that Cerion uva is 

 also an adept climber during the rains, but appeal's to migrate 

 down to fasten itself near the base of the trees and brush dur- 

 ing the dry periods. Drymaeus virgulatus, on the other hand, 

 commonly glues itself to the highest branches ; this is perhaps 

 related with its restriction to the more protected stations. 

 Finally, the subarboreal species of Tudora wander up the trees 

 in the rainy weather, but have not developed the opposite 



