natural, history

ACKNOWLEDGEMENTSI am deeply indebted to Dr. TORBEN WOLFF for the opportunity to study and reportupon the material collected by the "Galathea" Expedition on Rennell and GuadalcanalIslands. Also my sincere thanks are due to RALPH W. MINER, formerly of theAmerican Museum of Nat~ral History for permission to work up the materialcollected by the Whitney Expedition, and to his successors, WILLIAM EMERSO~ andFREDERICK WEIR for their continued interest in supplying data and material from theSolomon Islands. My thanks are also due to Dr. R. K. DELL of the Dominion Museum,Wellington, New Zealand, for the loan of specimens collected on Rennell andBellona Islands, Solomon Islands.ZOOGEOGRAPHICAL REMARKSThe existance of the endemic genus QUirosella on Rennell is certainly understandableif we accept the time factor and isolation. The occurrence, however, of three ratherwell defined species in this genus on this Island is far more difficult to understand.These three endemic species on Rennell Island occupy the same ecological niche, atleast in certain stations they were all found together while in others they occurredsingly. Rennell Island is small and its relief is low and the general ecology fairly uniform.This is in no way a unique example. The Island of Kauai in the Hawaiian Islandspossesses several genera in a family of mollusks, the Amastridae, known onlyfrom the Hawaiian Islands. The endemic genus Carelia has evolved several specieson this island and its small associated island Niihau. Certainly Kauai is very differentfrom Rennell. It is a "high island" of volcanic origin and presents a multitude ofphysiographic differences not found on a raised coral atoll. Nevertheless, the differenceis probably more apparent than real. Isolation on the higher elevations of thisisland has probably played its part in segregation. On the other hand, low landareas spread about the island making accessible most of the valleys and the ridges.Carelia is a ground species, living in the leaf litter on the forest floor.Perhaps another answer is suggested. It is possible that QUirose/la is a relict genuson Rennell and may at one time have occurred on other islands in the Solomons. It isalso possible that it still does exist on one or more of the islands but has so far eludedthe collector.It is important to realize that all of these islands of the Solomon Archipelago arestill very imperfectly known and many existing anomalies may be erased when theislands have been more thoroughly explored.However, there are two rather important points which are indicated by the landmollusks of the Solomon Islands. These are the relatively high endemicity of molluskson San Cristobal (at least two subgenera which include several peculiar species) andChoiseul Strait (between Choiseul and Bougainville) marking a very definite barrierin distribution of many groups of land mollusks.The first of these would seem to indicate that San Cristobal has been isolated for156