THE NAUTILUS. AL 
gills, the branchiz when distended with them being perfectly smooth 
outside, and looking like pads. There seem to be two great groups 
of these forms, one characterized by simple, oval or oblong shells 
destitute of any strong sculpture, and probably carrying the young, 
as a rule, only in the outer branchiz, and this includes in the 
United States such forms as Unio gibbosus Bar., U. tetralasmus Say, 
U. buckleyi Lea, U. crassidens Lam. and U. complanatus Sol., and 
these are probably closely related to the European species. The 
other group has short, rather solid, often inflated shells, with a wide, 
heavy hinge plate, and it includes nearly all the pustulous, and all 
the plicate sculptured forms. Lea found the inner and outer gills 
filled with embryos in four of these species: U. multiplicatus Lea, 
subrotundus Lea, kleinianus Lea, and rubiginosus Lea, and it is 
probable that, under favorable conditions, all or most of these spe- 
cies carry young to some extent, in the inner as well as the outer 
gill, though so far as I have observed the inner gill is never so com- 
pactly filled as the outer, and it is quite probable that with unfav- 
orable conditions the former may not be used as a marsupium. 
Besides these there are a few aberrant forms which may be, as 
Wetherby has suggested, “ geological remnants,” such as Unio 
phaseolus Hild., U. irroratus Lea, and U. cornutus Bar., having 
remarkable modifications of the branchial uterus or marsupium. 
These three species will probably have to stand as the types of as 
many genera. 
But little is known concerning the anatomy of the foreign Unios. 
The soft parts of all the European species have been examined, I 
believe, and descriptions which go into the minutize, so far as color 
and trifling peculiarities of form are concerned, have been pub- 
lished, but which give no idea of vital characters or structure. 
From all tbat I can learn the anatomy of the European forms is 
very much like that of the cireumboreal Unio margaritiferus, which 
is much like that of Unio gibbosus, crassidens, tetralasmus and the 
like. Of the Oriental and African forms I know almost nothing. 
I have examined the soft parts of gravid specimens of Unio gabo- 
nensis Kuster from Tropical West Africa, and found that in them 
the embryos filled the inner branchiz alone. 
It has been surmised that there was a close relationship between 
the Australasian Unios and those of South America. The shells of 
the species of the two faunas agree very closely in all characters ; 
in being destitute of rays, and having a uniform olive-green epider- 
