110 University of Michigan 



Practically all of the molluscs of the islands show a marked 

 preference for limestone rock, and most of them do not occur 

 in localities where the soil is non-calcareous. Bnjmaeus virgu- 

 latus, Succinea gyrata and Guppya molengraaffi are the con- 

 spicuous exceptions to this general rule. The first two appear 

 to be mainly dependent on the development of trees and brush, 

 while the last was only found near the summit of Sint Chris- 

 toffelberg (Cb20). Although several species invade the more 

 heavily wooded portions in the higher hills of the older rocks, 

 they are much rarer in these places than on the limestone. 

 The deep aestivation of Tudora fossor and the loss of sculp- 

 ture in Cerion uva have already been correlated with this 

 species. 



In addition, the fauna is characteristic of coastal conditions. 

 Cerion, especially, is usually limited, throughout its distribu- 

 tion, to the near vicinity of the seashore. The highland rain- 

 forest elements of both the Antilles and South America have 

 not obtained a footing on the present islands, although Guppya 

 molengraaffi appears to be limited to the region where such 

 conditions are most nearly approached. In places where such 

 a mesophytic fauna is present, its more hardy members may 

 invade the coastal region, but in the Dutch Leeward Islands 

 the opposite is true, and even the cerions reach to the highest 

 altitudes (1,200 feet). 



As a result of these ecological limitations, the fauna is only 

 comparable to that of similar regions in the Antilles and on 

 the mainland. This type of habitat has been quite thoroughly 

 studied in the former, but, in South America, collectors have 

 directed most of their attention to the richer inland forests. 

 A detailed study of the calcareous portion of the shore zone of 

 northern South America (c/. W. Sievers; 1896; I. c, pi. X) 

 might add considerably to our knowledge of the Antillean 

 elements in this region. On the basis of the present data, it 

 can only be stated that Aruba, as well as the other Dutch Lee- 

 ward Islands, is markedly isolated ecologically from the known 

 portions of the mainland. The similarity of the fauna of 

 Aruba and that of the other islands and its lack of resemblance 



