UNIONID. 241 
in being single in each valve, and tubercular, not entering a pit 
in the opposite valve, that of the right valve being posterior to 
the left. The typical group comprises South American species; 
the following subgenera, proposed for Asiatic species, have very 
slight distinctive characters. 
PSEUDODON, Gould, 1844. The hinge-tooth of the right valve 
fits into an emargination situated in front of the beak of the 
left valve, while the tooth itself of this latter valve is below the 
beak. M. Peguensis, Anthony. Burmah. 
TRIGONODON, Conrad, 1865. The hinge-tooth of the right valve 
erect, almost vertically or obliquely elongated, fitting into a 
divided tooth of the left valve, the posterior portion of the left 
tooth being much larger than the anterior. J. crebristriata, 
Anthony. Pegu. 
LEGUMINATIA, Conrad, 1865. Outline approaching Margaritana 
margaritifera, being medially contracted and of an oblong and 
leguminous shape. The cardinal tooth is pyramidal and recurved, 
wholly unlike the transverse compressed tooth of Pseudodon. 
M. Mardinensis, Lea. River Tigris. 
Dr. Stoliczka, commenting upon the above groups, concludes 
thus: 
“Large series of all these shells in different stages of growth 
should be examined, for there is no doubt that they gradually 
pass one into the other, and connect Margaritana with Unio. 
Certainly nothing can be more unnatural than creating new 
generic groups upon the examination of single shells, particu- 
larly among such most variable forms as the fresh-water Unionidz 
are known to be, and then attempting to assimilate the various 
species to the imaginary characteristics. It is just the contrary 
way, we have to take in a natural classification.” 
MICROCONDYL@A, Vest. Gills united to the mantle in their 
entire length, a small cardinal tubercle in each valve. JMarga- 
ritana Bonelli, Fer. Europe. 
Practropon, Lea, 1856. 
Distr.—P. Isocardioides, Lea (exxiv, 91). Rio Plata. 
Shell inequivalve, ventricose, obliquely trigonal ; cardinal teeth 
transverse, crenulated, compressed, double in each valve; no 
lateral teeth. 
Diesas, Leach, 1814. 
Syn.—Barbala, Humphrey, 1797.  Dianisotis, Raf., 1831. 
Symphynota, Swains., 1840. 
Distr.—2 sp. China. D. plicatus, Leach (exxiv, 92). 
Shell usually alate or winged postero-dorsally ; hinge witha 
single long linear tooth on the dorsal margin. 
