240 UNIONIDA. 
Genus Margaron, Lea. 
Subgenus Unio, Retzius. 
Symphynote— 
Plicate. 
Smooth. Triangular, oval, oblong. 
Non-Symphynote— 
Plicate. Quadrate, triangular, oval, oblong, subrotund, 
wide, arcuate. 
Nodulous. (Subdivided as above under “ Plicate.”) 
Spinous. Triangular. 
Suleate. Triangular, oval, oblong, subrotund. 
Smooth. Triangular, oblique, oval, oblong, subrotund, 
wide, obovate, arcuate. 
MARGARITANA, Schumacher, 1817. 
Syn.—Baphia, Meuschen, 1787. 
Distr.—40 sp. N. America, Europe. Fossil. Cret.—; N. Am. 
Shell like Unio, but without lateral teeth. 
There is so little difference between this genus and Unio, that 
for systematic purposes it would be better placed as a subgenus 
of the latter: the universal custom of using its generie name in 
connection with the species prevents such disposition of it. 
Dr. Lea applies to the classification of this genus the same ad- 
mirable scheme which he has used for the Uniones. H. and A. 
Adams restrict the typical group to those species having smooth, 
oblong, transverse shells, and then admit the three following 
subgenera, which might better be suppressed, and their names 
added to the generic synonymy. IM. margaritifera, L. (exxiv, 
88, 89), the species which afforded the once-famous British pearls, 
is circumboreal in distribution, occurring in Europe, Siberia, 
British America and the Northern United States. It is a re- 
markable exception to the rule of limited distribution of the 
species of the family. 
ALASMODONTA, Say, 1820. Shell subtrigonal, inflated, beaks 
prominent, surface smooth. J Curreyana, Lea. 
COMPLANARIA, Swains., 1840. Valves connate, surface plicate. 
M. confragosa, Say. 
UNIOPSIS, Swainson, 1840. (Calceola, Swainson, 1840.) Shell 
ovate; surface of valves smooth ; cardinal teeth two, irregular 
or receding from the anterior margin, or with tubercles and 
undulations representing cardinal teeth. MM. caleeola, Lea. 
Monocondyta@a, d’Orb., 1835. 
Distr.—30 sp. South America, Southern Asia. MM. Guaray- 
ana, d’Orb. (exxiv, 90). 
Shell externally like Unio,internally the hinge, like Margaritana 
possesses cardinal but not lateral teeth; the cardinals are peculiar 
