UNIONIDZ. 239 
HYRIDELLA, Swains., 1840. (Micromya, Agassiz, 1852.) Shell 
oval, smooth. JU. batavus, Lam. (cxxiii, 78). 
OBOVARIA, Rafinesque. Shell subrotund, smooth. JU. retusus, 
Lam. (cxxiii, 79, 80). 
LAMPSILIS, Rafinesque, 1820. (Pleurobema, Plagiola, Scale- 
naria, Syntoxia and Truncilla, Rafinesque. glia, Swains., 1840.) 
Shell more or less triangular or oblique, truncated posteriorly, 
smooth. U. elegans, Lea (cxxiii, 81). 
CANTHYRIA, Swainson, 1840. Shell subtrigonal, spinose. U. 
spinosus, Lea (exxiii, 82). 
IRIDEA, Swainson, 1840. Shell oval or wide, nodulous. U. 
cylindricus, Say (exxili, 83). U. perplexus, Lea. 
ROTUNDARIA, Rafinesque, 1830. Shell obliquely rounded, nod- 
ulous. U. pustulosus, Lea. 
QUADRULA, Rafinesque, 1820. (Theliderma, Swainson, 1840.) 
Shell subquadrangular, tuberculated. U. apiculatus, Say. 
DIPLODON, Spix, 1827. Shell oval, oblong or wide, plicate. U. 
pliciferus, Lea. U. Grayanus, Lea. 
DYSNOMIA, Agassiz, 1851. Shell triangular or quadrangular, 
plicate. U. plicatus, Lesueur (exxiii, 84). 
METAPTERA, Rafinesque, 1820. Shell with a wing-like postero- 
dorsal prolongation of the valves, by which they become connate 
or soldered together. U. alatus, Say (exxiii, 85). U. delphinus, 
Gruner. 
UNIOCARDIUM, Capellini, Obliquely lengthened, very inequi- 
lateral, the margin produced below, with a posterior sharp rib or 
angle. Fossil. Congarienschichten, Sterza. 
unrona, Pohlig, 1880. Shell thick, somewhat inequivalve, 
with a lunule; apex eroded; hinge with two three-cornered teeth 
in the right valve and one in the left valve, right valve with one 
short lateral tooth, left with two; near the anterior muscular 
impression are two accessory impressions. 2 sp. Triassic; 
Germany. U. maritima, Pohlig. 
LOXOPLEURUS, Meek, 1876. Elongate-subovate, somewhat 
arcuate, without wings, ornamented with two sets of very regu- 
lar, well-defined costz, those of one of which start from the 
margin just in front of the beaks, and radiate obliquely back- 
ward and downward; while those of the other set start from the 
dorsal margin behind the beaks, and extend downward in a 
direction that would cause them to intersect those of the other 
series at rather acute angles along the posterior umbonal slopes 
somewhat asinGoniomya. U.belliplicatus,M. Fossil. Wyoming, 
near junction of Cret. and Lower Tertiary. 
The following arrangement by Dr. Lea is based upon the same 
characters as the above “subgenera,” but is a more comprehensive 
scheme, providing pretty well for the natural grouping of all 
the species. 
