Occasional Papers of the Museum of Zoology 113 



hypothesis. The question must be left in abeyance, at least 

 until a thorough examination of the remainder of the Leeward 

 Islands and the northern coast of South America has been 

 made. 



Distribution within the Islands 



As most of the species of the Dutch Leeward Islands are 

 almost entirely limited to the limestone rim, breaks in this 

 establish quite definite barriers to intermigration. Especially 

 on the island of Curasao, the sunken valleys permit penetra- 

 tion by arms of the sea, so that the rim is cut up into a series 

 of quite isolated ridges. As a result, the facies of certain 

 species of the land shells changes slightly at each "baai" or 

 "lagoen. " After several successive breaks, the species of 

 Tudora, especially, often change so much that there is no inter- 

 gradation between lots from the colonies so separated. It is 

 on such changes that the various subspecies are based. 



The largest island, Curagao, can be divided into three, quite 

 distinct, faunal areas, which appear to have been populated 

 from central hills, which must have been separate islands 

 during periods of higher strand-line. Aruba, on the other 

 hand, is almost a unit, although Spaansch Lagoen roughly 

 coincides with a slight break in the molluscan fauna (see Plate 

 I, map of Aruba). Bonaire and Klein-Bonaire together form 

 a similar unit. 



The most southern area of Curasao centers around the 

 Tafelberg of Santa Barbara (C2). Tudora ruins rupis, T. 

 pilshryi and BracJiypodella ravcni sancfaeharharae were only 

 found at the base of the northern and western escarpments 

 of this mesa. The first species is also represented by the sub- 

 species neivportensis near New Port (CI), although the re- 

 mainder of the fauna of the shore zone is more like that of 

 central Curasao. In addition, Cistulops raveni appears to be 

 absent from southern Curagao. 



The central area (CI, 3-12) may be considered to center 

 around Ronde Klip (C12) and the Hato ridge (Cll). It is 

 characterized by the absence of the section Tudorata, and by 



