PROCEEDINGS OF THE NATIONAL MUSEUM. 



often a few nodulous or wavy ridges cross the body of the sliell, 

 which is irregularly sulcate; epidermis rayless; two pseudocardinals 

 in the right valve, the upper compressed, separated from the split-up 

 lower one by a parallel-sidedpit, two in the left, and two heavy laterals 5 

 one lateral in the right and a vestige of one below itj laterals granu- 

 lated and vertically striate; muscle scars deep, anterior small, the front 

 and upper united, the hinder separate, posterior muscle scars long, 

 oblique; beak cavities very shallow; dorsal scars numerous, placed 

 just on the inner edge of the hinge; nacre white. The lobes of the man- 

 tle are not united into siphons, but separated throughout, with only a 

 feeble commissure separating the anal and branchial, openings. 



(Groux) of Arconaia lanceolata.) 



Shell with an anterior wing. 



t ARCONAIA LANCEOLATA Lea. 



* Triquetra lanceolata Lea, Pr. Ac. N. Sci. Phila., VIII, 1856, p. 79. 

 ^ ffi/ria lanceolata Lea, Pr. Ac. N. Sci. Phila., 1856, p. 300. 



"" Arconaia lanceolata Coxrad, Am. Jl. Couch., 1, 1865, p. 231. 



''Triquetra coniorta Lea,' Pr. Ac. N. Sci. Phila., VIII, 1856, p. 300; ^Obs., VI, 1857, 



p. 39, pi. XXXIII, fig. 33; *J1. Ac. N. Sci. Phila., Ill, 1858, p. 319, pi. xxxiii, 



fig. 33. 

 ""Eyria co«<o?-to Sowerby, Conch. Icon., XVII, 1869, pi. i, figs. 2 a, 2&.— *P.etel, 



Conch. Sam., Ill, 1890, p. 189. 

 ''Margaron (Triquetra) coniorta Lea, Syn., 1870, p. 26. 

 ' Unio oontortas Heude, Conch. FIut. Nank., II, 1877, pi. xv. fig. 31. 



China. 



t ARCONAIA MUTICA Heude 



* Unio contortus var. mutlcus Heude, Conch. Fluv. Nank., II, 1877, pi. xv, fig. 32. 



China, 



(Group of Arconaia conjungens.) 



Shell moderately solid, slightly twisted and bent, rounded in front; 

 pseudocardinals not so much torn as in the lanceolata gronp ; not winged 

 in front. 



ARCONAIA CONJUNGENS Heude. 



* Unio contortus var. conjungens Heude, Conch. Fluv. Nank., II, 1877, pi. xv, fig. 33. 

 China. 



Genus PSEUDAVICULA Simpson, 1900. 

 (Type, Unio johnstoni Smith.) 



Shell rather solid, inflated, with a well- developed anterior and pos- 

 terior dorsal wing ; dorsal line strongly incurved ; beaks full, not high ; a 



'The specific name was changed to contorta by Lea, because he thought his earlier 

 name, lanceolata, was less appropriate. 



