PROCEEDINGS OF THE NATIONAL MUSEUM. 



DIPLODON VARIABILIS Maton. 



* Ifya rarlaliUs MATON, Tr. Linn. Soc. Lond., X, 1811, p. 327, pi. xxiv,figs. 4-7. ' 

 MVooD, Gen. Conc-h., I, 1815, p. 111. *DiLi/WYN, Cat., I, 1817, p. 53. *Woon, Ind. 

 Test., 1825, p. 13, pi. 11, fig. 38a; rev. ed., 1856, p. 17, pi. in, fig. 38. 



* Unio variabilis D'Ommixv, Voy. Am. Mer., 1843, p. G04, pi. LXXi, figs. 1-3. 



*HAXLEY, Test. Moll. ,1842, p. 202; *Biv. Shells, 1843, p. 202. *CATLOW and 

 EEKVE, Conch. Noui., 1845, p. 65. * H. and A. ADAMS, Gen. Rec. Moll., 

 II, 1857, p. 492. *SOWEKHY, Conch. Icon., XVI, 1868, pi. LXXIV, fig. 381. 

 * STROBEL, Mat. Mai., Pt. 1, 1874, p. 69.* P.ETEL, Conch. Sam., Ill, 1890, 

 p. 171. 



* Margarita (Unio) variabilis LEA, Syn., 1836, p. 33; 1838, p. 23. 



* Mar gar on ( Unio) rariaUUs LEA, Syn., 1852, p. 35; 1870, p. 56. 



* Mytilus matoniana D'OiuiiGNY, Guer. Mag., 1835, p. 35. 



* Unio matonianus P.ETEL, Conch. Sam., Ill, 1890, p. 158. 



* ? Unio membranacea HANF.EY, Biv. Shells, 1843, p. 202, pi. xxn, fig. 6. 



* Unio membranaceus PHILIPPI, Conch., Ill, 1848, p. 80, pi. v, fig. 4. * KUSTER, 



Conch. Cab. Unio., 1862, p. 284, pi. xcv, fig. 5. 



Eio de la Plata. 



Subgenus HYRIDELLA Swainson, 1840. 

 (Type, Unio ausiralis Lamarck. ) 



Beaks rather low, sculpture consisting of curved, generally nodulous 

 ridges, which approach below but usually have a smooth area of shell 

 between them; surface sulcate or sometimes corrugated and nodulous; 

 epidermis rayless; teeth rather delicate, compressed, often somewhat 

 rudimentary. 



Animal having the embryos occupying the inner gills for the most 

 part, 2 which are united for their entire length to the abdominal sac; 

 outer gills pointed below in the middle; palpi triangular; branchial 

 opening papillose; anal opening smooth, not separated from the super 

 anal opening. 



Section HYRIDELLA Swainson. 1840. 



Shell covered with concentric, sulcate sculpture; pseudocardinals 

 well developed, compressed; laterals delicate, sometimes imperfect. 



(Group of Diplodon dorsuosus.) 



Shell somewhat rhomboid, inflated solid, with rather full beaks, the 

 sculpture consisting of very strong, subradial, corrugated ridges which 

 curve toward each other below, those in the center sometimes joining, 

 this sculpture extending well out on the disk; upper part of pos- 

 terior slope having slight radial folds. 



'I feel certain that Maton has illustrated two species, figs. 4, 6, and 7 being nearly 

 circular, with a rather straight hinge, while fig. 5 is a small shell, is much inflated 

 at the posterior base, and is incurved in front of the breaks. He describes the large 

 shell first. The small one may be the young of D. palelloides. 



2 According to Suter in a letter. 



