95 



OBSEEYATIONS ON THE VAEIABILITY OF THE 

 TASMANIAN VNIO. 



By E. M. Johnston, F.L.S. 



Having collected many specimens of the genus TJnio 

 inhabiting the northern rivers of Tasmania, during the last 

 seventeen years, more especially those found in various parts 

 of the South Esk Eiver, I have often been much impressed 

 with the extreme variability of form and colour exhibited by 

 different individuals. This is more particularly remarkable 

 if specimens marking different stages of growth are compared 

 with each other. 



If specimens marking seven successive stages of growth be 

 compared together as in the plate accompanying this paper, it 

 will be observed that the variation in form — from youth to 

 the adult stage — embraces characteristics which cover most 

 of the distinctions upon which many of the Australian forms 

 mainly depend for the recognition of distinct specific rank. 

 Nor is this variability confined to the form of the shell. In the 

 first four stages of growth the examples collected by me near 

 Carrick, on the South Esk, correspond in nearly in all respects 

 with TJ. Wilsoni (Lea), as figured and described by Eeeve 

 (fig. 472), i.e., " Shell thin, rather depressed, eld2:>tic, oblong, 

 somewhat retuse below, with delicate and concentric grooves, 

 shining, olive green, obscurely rayed (some examples only) ; 

 umbonas ridge rounded and scarcely raised ; beak a little 

 prominent and not sculpturic ; nacre, bluish white ; primary 

 teeth small, oblique, lamellar ; lateral teeth, long, straightish." 



Among these stages of growth some are to be found which 

 are with difiiciilty distinguished from U. Stuartii, Adams and 

 Angus, especially in its young stage. 



Many of the individuals of the fourth and fifth stages of 

 growth agree in most resjiccts witli U. Nt'peanensis, Conrad, 

 while the individual variations of the adult or sixth and 

 seventh stages, embrace generally all the characteristics of the 

 following Australian forms, viz. — 



Unio Australis. Lamarch 



depressus. Lamarch 



ambiguus. Parreys 



Balonensis. Conrad 



Phillipianus. Kuster 



Moretonicus. Meeve 



Vittatus. Lea 



If such be the variability of our local form in the indivi- 

 duals of the various stages of growth, there is good reason for 

 the belief that the several forms erected into specific ranks in 



