21 



gradually flattening towards the pointed posterior end ; 

 diagonal ridge inconspicuous, rounded ; posterior slojoe, 

 small ; umbones large, broad, becoming somewliat flattened 

 by decortication ; shell suljstance moderately thick ; surface 

 coarse and rough on the anterior end, with strong, 2:»rominent, 

 concentric lines of growth, which gradually flatten out into 

 laminsc on the posterior end ; bent upwards at the rounded 

 diagonal ridge ; no sign of radiatory lines ; dental and mus- 

 cular characters unknown. Length, 3 in. 10 lines ; breadth, 

 2 in.; depth of the united valves, 1 in. 10 lines. 



Ohs. — The condition of the specimens, both in this and the 

 succeeding sj^ecies, renders it very difficult to give some of 

 the characters with accuracy. This is especially the case 

 with the surface ornament, wliich becomes much obliterated. 



I have carefully compared these shells with the fino 

 collection of recent TInionidce in the British Museum, and 

 cannot suggest any species with which they may be com- 

 pared, unless it is tfnio trimidus, E-itzius. 



There are many produced species of Unio, such as U. 

 Buddianus, Lea ; tl. She^ardicuiMS, Lea ; or U. rectus. Lamk.^ 

 but with neither of these are there characters in common. 

 In the massive convex form our species apj^roaches UniO' 

 Anodontoides, Lea, from Alabama, but accuminates posteriorly 

 too rapidly, and has not the ventral sinuation of that species. 

 Similarly, it is too convex on the anterior side of the shell 

 for TJ. Gihhosus, Burns, a ISTorth American form, and does 

 not possess the well-marked diagonal ridge of the latter. 



U. Johnstoni does not resemble any of the living Australian 

 species with which I am acquainted, but approaches nearest 

 to JJ. mutabilis, Lea, which is found around Brisbane, 

 although the resemblance here is of a slight character. Ac- 

 cording to the most recent investigations amongst the land 

 and freshwater shells of Tasmania, by the Eev. J. E. T. 

 Woods* and Mr. R. M. Johnston, f only one species of Unio 

 is still known to exist there. Both these authors quote Z7. 

 Moretonicus, Sow., as occurring in the northern rivers of 

 Tasmania, and a comparison of this with TJnio Johnstoni is 

 unnecessary, the dissimilarity between the two shells is so 

 great ; but here arises the interesting fact that Tasmania, at 

 the present day, should possess only one sj^ecies of TJniOy 

 whilst in Tertiary times it is possible that several existed. 



I have not been able to meet with any described fossil 

 Unio with which the present species need be compared. 



* Ou the Freshwater Shells of Tasmania (Papers and Proc. II. Hoc. Tas.), 

 1875, 8vo., p. 17. 



t Further Notes on tlie Freshwater Shells of Tasmania (Papers and 

 Proc. li. Hoc. Tas.), 1877, 8vo., p. 11. 



