20 



obtained in myriads on the submerged leaves of Triglocliin 

 and Potamageton. Indeed, varieties may be found abun- 

 dantly wbicb sliade off impercej)tibly from one to tbe otlier 

 of tbe cbief varieties. Again, size and degree of transparency, 

 whether due to the shell or its covering, are very deceptive, 

 as I find that, in the same neighbourhood, a particular 

 species, as it approaches brackish or salt water, becomes 

 thinner, more transparent, and, frequently, reduced to half 

 the fresh water size. The aspect of certain shells is also 

 much affected by local causes. Thus, shells in still water, 

 fresh, where much dead wood is found, are invariably covered 

 with a dark coat, and the shell is usually more solid. Where 

 the water j^lants, Triglochin and Potamageton, abound, the 

 shell becomes coated with a rusty brown color from decom- 

 posed vegetable matter. 



Those shells which adhere to stones more or less exposed 

 to the direct rays of the sun, are frequently bleached white, 

 and become more solid in shell structure, and sometimes 

 very brittle. 



In the 1st Basin, Launceston, I have noticed that a small 

 shell, which answers essentially to the description Bithynella 

 Legrandi (Tenison-Woods), mimics closely the color of the 

 . rock upon which it adheres. There generally it is found 

 with a white shelly lip. Perhaps this mimetic tendency may 

 help to explain the extreme variability of the genus Bithynella, 

 which is undoubtedly the most prolific fresh water genus in 

 Tasmania. It is also the most generally distributed shell, 

 and is found from the sea to 3000 feet above its level. 

 PhyscB and Pisidia, however, have a greater vertical range, as 

 certain species of Physa und Pisidium abound in Youl's Lake, 

 Ben Lomond, 5070 feet above sea level. The species are, 

 however, very stunted at that height. 



Although I shall j^resently describe one or two peculiar 

 and remarkable shells — one of them, at any rate, of a rare 

 genus hitherto known as confined to America and Cuba — 

 yet, when we consider the wonderful minuteness of fresh 

 water shells, the paucity of collectors, and the vast regions 

 yet unexplored in Tasmania and Australia, it would be 

 unwise to make wide generalisations upon our limited col- 

 lections, however tempting it may be for individual ob- 

 servers to so. As yet, I believe, we are a long way off that 

 completeness of knowledge of the fresh water shells of 

 Tasmania to justify anyone in considering that our fresh 

 water shells are exceptionally distinct from neighbouring 

 regions, where, possibly, the same lack of knowledge also 

 prevails. 



As an illustration of how a classifier may be misled by 



