UNIONID^. 239 



HYRiDELLA, Swains., 1840. (Microm^^a, Agassiz, 1853.) Shell 

 oval, smooth. U. batavus, Lam. (cxxiii, 78). 



OBOVARiA, Rafinesque. Shell subrotund, smooth. U. retusus, 

 Lam. (cxxiii, 19, 80). 



LAMPSiLis, Rafinesque, 1820. (Pleurobema, Plagiola, Scale- 

 naria, S3mtoxia and Truncilla, Rafinesque. ^glia. Swains., 1840.) 

 Shell more or less triangular or oblique, truncated posteriorly, 

 smooth. U. elegayis, Lea (cxxiii, 81). 



CANTHYRiA, Swainson, 1840. Shell subtrigonal, spinose. U. 

 spinosus^ Lea (cxxiii, 82). 



IRIDEA, Swainson, 1840. Shell oval or wide, nodulous. U. 

 cylindricus, Say (cxxiii, 83). U. perplexiis, Lea. 



ROTUNDARiA, Rafincsque, 1830. Shell obliquely rounded, nod- 

 ulous. U. 2:)iistulosus, Lea. 



QUADRULA, Rafiuesquc, 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 (cxxiii, 84). 



METAPTERA, Rafinesquc, 1820. Shell with a wing-like postero- 

 dorsal prolongation of the valves, by which they become connate 

 or soldered together. U. alatus, Say (cxxiii, 85). U. delphinus, 

 Gruner. 



UNiocARDiUM, Capelliui. Obliquely lengthened, very inequi- 

 lateral, the margin produced below, with a posterior sharp rib or 

 angle. Fossil. Congarienschichten, Sterza. 



UNIONA, 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 costse, 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 as in Goniomya. 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. 



