22 THE NAUTILUS. 



mis and a slightly concentrically sculptured surface, simple out- 

 lines, rather dull, bluish- white nacre, compressed cardinals and im- 

 perfectly radial beak sculpture. Lea examined gravid specimens 

 of Unio peculiaris Lea, and firmus Lea from South America, and 

 found that only the inner gill was filled with embryos. Suter 

 reported the same thing from an examination of Unio menziezi 

 Gray from New Zealand. I recently received some fine alcoholic 

 specimens of that species from him, and on examining them found, 

 to my astonishment, that they agreed with Lea's descriptions of the 

 soft parts of the South American forms as exactly as if they were 

 the very animals that he had described. In all three species the 

 outer gill is greatly produced below in the middle, the anal opening 

 is destitute of papillae, and there is no super anal opening at all, 

 characters which are conspicuous in the South American species. I 

 had previously placed these Australasian and South American 

 Unios in a subgenus by themselves, for which I used the name 

 Diplodon, applied by Spix to some Brazilian forms, 6 but I am satis- 

 fied that they are entitled to generic rank, and Spix's name may be 

 used for the group. I do not believe that they belong to the same 

 phylum with the Unio gabonensis which, from conchological charac- 

 ters, seems much more closely related to the forms of Southern 

 India. This seems to add another link to the chain of evidence 

 which goes to prove a relationship between the faunas of Australa- 

 sia and South America, and it is a question whether this relation 

 came about on account of migration, by way of an Antarctic laud 

 way from one continent to another, or whether the two faunas are 

 remnants of an earlier and generally distributed northern fauna that 

 was driven south and superceded by more modern forms. The 

 Unios of South America and Australasia are simple forms, both 

 anatomically and conchologically. Long ago Ihering predicted 

 that the earliest Unios would be found to have radial beak sculp- 

 ture ; and two of the fossil species recently described by the 

 writer 7 from what are supposed to be the Triassic freshwater beds 

 of Texas have that which is strictly radial. In the Australasian 

 and South American forms the beak sculpture is imperfectly radial, 

 the central rays curve together and generally coalesce, and in some 



6 The Classification and Geographical Distribution of the Pearly Fresh- 

 water Mussels. Proc. U. S. Nat. Museum, XVIII, 1896, p. 302. 



7 Description of Four New Triassic Unios from the Staked Plains of Texas. 

 Proc. U. S. Nat. Museum, XVIII, 1896, pp. 381-385. 



