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 t3'pical group comprises South American species; 

 the following subgenera, proposed for Asiatic species, liave 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, Courad, 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. M. crehriatriata^ 

 Anthony. Pegu. 



LEGUMINAIA, Conrad, 1865. Outline approaching Tfefargfariiawa 

 nxargai-itifera^ being medially contracted and of an oblong and 

 leguminous shape. The cardinal tooth is p^-ramidal 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. 

 Certainl}'^ nothing can be more unnatural than creating new 

 generic groups upon the examination of single shells, particu- 

 larl}'^ among such most variable forms as the fresh-water TJnionidfe 

 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." 



MiCROCONDYLCEA, Vcst. Gills United to the mantle in their 

 entire length, a small cardinal tubercle in each valve. Marga- 

 ritana Bo7ielli, Fer. Europe. 



Plagiodon, Lea, 1856. 



Distr. — P. Isocardioides, Lea fcxxiv, 91). Rio Plata. 



Shell inequi valve, ventricose, obliquely trigonal ; cardinal teeth 

 transverse, crenulated, compressed, double in each valve ; no 

 lateral teeth. 



Dipsas, Leach, 1814. 



Syn. — Barbala, Humphrey, 1197. Dianisotis, Raf., 1831. 

 Symphynota, Swains., 1840. 



Distr. — 2 sp. China. D. plicatus^ Leach (cxxiv, 92). 



Shell usually alate or winged postero-dorsaily ; hinge with a 

 single long linear tooth on the dorsal margin. 



