PROCEEDINGS OF THE NATIONAL MUSEUM. 331 



13ustnlous species, in which one valve appears as if it had beeu puslied 

 downward when in a plastic state, and is always less inflated than the 

 opposite one. 



These peculiarities are not characteristic of entire groups, as they 

 may be met with in one species and absent in closely related forms. 



The Australian Region. — Australia, Tasmania and IsTew Zealand are 

 peopled with a very distinct set of ]SIaiades, consisting, with the excep- 

 tion of the single Solenaia which has been referred to the former 

 island, of Unios only. It may be possible that when Kew Guinea is 

 thoroughly explored, some of the peculiar species of Fnios found in 

 Australia may be discovered, as it is believed that these two islands 

 were connected during Tertiary time. Only a moderate number of 

 species are found in this region, as Australia has few streams, and all, 

 or nearly all, of them either go dry or are reduced to mere disconnected 

 pools in time of drought. In general, the shells of this region are oval in 

 outline, smooth, of a dull greenish olive or brownish tint, and without 

 other patterns of color marking. Some of the forms have a slight 

 development of concentric ridges, and only two species are known 

 which have any other sculpture: U. cucmnoides, Lea, which is some- 

 what tuberculous, and U. napeanensis, Conrad, which has rather sharp, 

 pointed knobs or corrugations, extending out for some distance from 

 the beaks. Unio dorsuosus, Gould, the type of which is in the Museum 

 collection (Xo. .5925), is, I have no doubt, a young U. napeanensis^ and 

 is said to have come from the Fiji Islands.^ 



At the beaks of this shell the sculpture is imperfectly radial, much 

 resembling that of the South American species. The very few perfect 

 beaks of Unios of this region which I have seen, have a somewhat zig- 

 zag or curved radial sculpture, indicating, as do the form and color of 

 the shells and the similarity of the soft parts, a close relationship with 

 the South American species. The so-called Alasmodonta sfuarfi, from 

 Australia, is merely a Unio with compressed, feebly developed teeth. 

 No si)ecies of this region is known to be extralimital, and the Solenaia, 

 if really from Australia, is the only member of a foreign group repre- 

 sented in this region. 



The Mississippi Region. — All the waters that are carried to the Gulf 

 of Mexico through the Mississippi River are filled with a common 

 assemblage of Xaiades, consisting of Unios and Anodontas. In fact, 

 this fauna occupies almost exclusively all the streams emptying into 

 the Gulf, from the Rio Grande on the west to the Chattahoophee River 

 on the east, and beyond this either the species of this region or those 

 belonging to its groups are scattered from Central America to North 

 Carolina. To the northward, other species or members of groups 

 belonging here have jiassed into New England and extended down to 



1 Gould says (U. S. Expl. Exp., XII, p. 431): "This shell was inartetl Fiji Islands, 

 probably by some accident, as I doubt not that it canie from eastern Asia." It is no 

 doubt an Australian, and not an Asiatic or Polynesian species. 



