Hy 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 papille, and there is no super anal opening at all, 
characters which are conspicuousin 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,° 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 land 
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was driven south and superceded by more modern forms.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’ 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 
® The Classification and Geographical Distribution of the Pearly Fresh- 
water Mussels. Proc. U.S. Nat. Museum, X VIII, 1896, p. 302. 
7 Description of Four New Triassic Unios from the Staked Plains of Texas. 
Proc. U. 8. Nat. Museum, XVIII, 1896, pp. 381--385. 
