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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 



