19 



FUETHER NOTES ON THE FEESH- WATER SHELLS 

 OF TASMANIA, 



(Witli a description of New Sj^ecies) 



BY 



EOBEET M. JOHNSTON. 



[Bead 12th March, 1877.] 



INTEODUCTOEY. 



The publication of the Eev. J. E. Tenison-Woods' paper, 

 in enabhng collectors to determine and compare the cata- 

 logued fresh water sj^ecies with those in their immediate 

 neighbourhood, has been most valuable, and has given a 

 healthy impetus to local observation. As the list, however, 

 was only published by the Society but a year ago, it cannot 

 be expected that any considerable addition could have been 

 made to our knowledge in this particular study since that 

 time. Still, as the types for the various species were mainly 

 derived from southern habitats, it is possible that some 

 varieties peculiar to northern Tasmania may have escaped 

 notice, and consequently there may be a difficulty in the 

 exact specific determination of certain of our northern 

 varieties. Indeed, I find, with respect to Fhysa, Lymnea, and 

 BitJiynella, that the difficulties in the way of classification are 

 very great, because of their extreme variability even in the 

 same habitat. This difficulty is, unfortunately, increased 

 where the full range of varia^bility may have escaped the 

 classifi.er's notice. Perhaps it is owing to this that Mr. 

 Brazier's two species — Paluclestrma Legrandiana and P. 

 Wisemaniana, so far as written description is concerned, 

 could not be identified by Mr. Woods, who could have 

 scarcely failed to recognise them from Mr, Brazier's descrip- 

 tion had he seen the most common species from northern 

 habitats. I am fully convinced, from a careful examination 

 of the living shells in the neighbourhood of Launceston, that 

 Paludestrina Legrandiana (Brazier), P. Wisemaniana (Ibid), 

 Bithynellaunicarinata (Tenison-Woods), B. Tasmaniciis (Ibid), 

 are simply the chief varieties of one species. As the classifier, 

 who has frequently to be content with dead specimens and a 

 meagre show of varieties, may find it difficult sometimes to 

 know what may constitute permanent characteristics, it may 

 be of some use to state my observations with respect to 

 certain characters which, in my opinion, are very inconstant. 



In the first place, I have noticed that the keel membrane 

 characteristic of 5. imicara^a^a (Tenison-Woods) and Palu- 

 destrina Legrandiana (Brazier) is not a reliable character. In 

 the swamp drains, in Launceston, the four varieties may be 



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