60 CONTRIBUTIONS FOR A SYSTEMATIC CATALOGUE OF THE AQUATIC SHELLS OF TASMANIA. By W. F. Pettebd. Part I. I purpose in a series of papers revising the somewhat large amount of work that has ah-eady been done, recording omissions, and describing newly discovered species and varieties of the fresh water shell-bearing mollusea of this island, preparatory to the compilation of a systematic catalogue in which the groups will be defined, the specific characteristics explained and geographical distribution recorded. Such a catalogue carefully criticised with the necessary bibliography will, 1 think, supply a desideratum much required by the general collector and may also be of service to the more philosophical student. All workers in this special field of zoology well know the extreme difficulty to be surmounted as to specific limitation from the great variability of aquatic testacea in all parts of the world. This is caused by a very large number of local influences retarding, or otherwise, the development of the more pronounced and important specific characteristics, so that many supposed distinct species collected from special localities prove not to be so when a large series are examined from many habitats ; on the other hand, it sometimes occurs that what are considered simple varieties prove to be specifically distinct when carefully compared with typical examples. The most apparent influences arc the greater or less rapidity of the streams in whicli they live, the chemical effect of the mineralogical formation through which they flow, the variety and more or less abundance of the requisite food-plants, combined with the varying altitude of the habitat ; all are important factors in jtroducing modification of the shell covering, but fortunately the animal is far less susceptible to viiriation. It is now a well established truth that its examination is an almost infallible guide for the determination of species, so that it becomes absolutely necessary to under-take an extensive series of comparisons from as many localities as are accessible before a systematic catalogue can he worked out so that it may be of real scientific value and service. The primary reason for ray recent investigations was to endeavour to discover the correct genus in the system of
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