648 SUPPLEMENT. 
points from mine, in which I merely endeavoured to unite the more nearly allied species 
into groups or subgenera according to the characters of the adult shell, in order to 
facilitate their determination. As regards the subgenera, the following points must 
be noticed :— 
All the Mexican and Central-American species of Unio, with the North-American 
and European (Palearctic) forms, are placed by Simpson in his subfamily Unionine, 
characterized by the concentric sculpture of the summits and the position of the 
embryos in the outer gills; none, on the contrary, in the subfamily Hyriane, with 
radial or zigzag-radial sculpture of the summits and the embryos in the inner gills 
only, comprising South-American, African (Ethiopian), East Indian, and Australian 
species. 
CrEeNoponta (pp. 479, 492). 
Simpson admits Crenodonta as a section of his genus Quadrula, division 'Tetragene. 
Psoronalas (pp. 480, 493). 
Simpson places most of the species of Psoronaias in the genus Unio, section Elliptio, 
Raf., division Homogene ; U. ostreatus and U. percompressus, however, are included 
by him in Quadrula, section Rotundaria. U. granosus, Brug., from Cayenne, is 
referred to the subfamily Hyriane, genus Diplodon (loc. cit. p. 878), because it is 
South-American. Unfortunately, I have not a specimen of U. granosus before me for 
comparison, but in the nearly allied U. coriaceus, Dunker (Zeitschr. fiir Malak. 1848, 
p. 181), from Rio Janeiro, the sculpture of the summits is very like that of the Mexican 
U. semigranosus (see Tab. XXX. fig. 1). It is true that in the Mexican species the 
oblique rows of granular elevations run chiefly in the direction from behind and above 
to before and below; in the Brazilian form they run in this direction in the hinder 
part of the summits only, and in the fore part from before and above to behind and 
below, which produces a convergence of the rows, as in Hyria corrugata. This may 
prove to be a reliable distinctive mark between the Mexican and the South-American 
Unionide with granulated sculpture? If so, Psoronaias is no exception to the rule 
that the Mexican and Central-American forms of the old genus Unio are completely 
separated from the South-American ones. 
Unio crocodilorum (p. 495). 
Var. rudis. 
Quadrula rudis, Simpson, Proc. Acad. Phil. 1900, p. 82, t. 8. fig. 2‘. 
Long. 103, alt. 65, diam. 45 millim. Summits in } of the length. 
Hab. Guatemaua: Rio Taxtunilha (Wheatley *). 
There is already a Unio rudus, Lea, but rudis (“raw”) and rudus (‘lump ”) have a 
different meaning in Latin. 
