SYSTEMATIC ARRANGEMENT OF THE NAIADES. 



Family UNIONID:^. 



Shell nacreous, with a thick epidermis; beaks usually sculptured, 

 often showing- the remains of the nuclear shell; ligament opisthodetic ; 

 hinge with or without teeth, though with vestiges of them in every 

 genus; when present schizodont and arranged as XDseudocardinals and 

 laterals ; palleal line usually simple ; prismatic border ordinarily narrow. 



Animal with labial iDalpi almost always wider than long ; anal opening 

 usually separated from the superanal; embryo a glochidium, the soft 

 parts being inclosed in a bivalve shell, and borne in the inner or outer 

 or all four leaves of the branchiae. 



SuTafamiljr TJlSriOnsriN'^^ S^vtlilasol^, 1840. (ExoTar-anohise.) 



Shell having essentially concentric beak sculpture. 

 Animal with labial palpi somewhat drawn out, projecting posteriorly j 

 embryos borne in the outer or in all four gills. 



HETEROGKN^^. 



Male and female shells different, the latter inflated in the post-basal 

 region; beak sculpture usually doubly looped; embryos contained in 

 ovisacs separated by a sulcus and occupying the hinder part of the 

 outer gills. 



Genus TRUNCILLA Rafinesque, 1819. 



(Type, Truncilla triquefer Eafiuesque.) 



Trimcilla RAFiNEsqvE, J. dePhys., Chimie, Hist. Nat., LXXXVIII, 1819, p. 427. 



Shell rounded or oval, solid, inflated, generally smooth and rayed, 

 with delicate beak sculpture which has a tendency to be doubly looped, 

 that of the female having a very decided inflation in the post-basal 

 region, which is thinner than the rest of the shell, of different texture, 

 often toothed, and usually radiately sculptured; laterals double in 

 each valve, the inner in the right valve smaller. Animal generally 

 having the inner gills united to the abdominal sac; female with a 

 heavy flap of the mantle which fills the post- basal swelling of the 

 shell, and which has an inner ridge inside at some distance above 

 the edge; marsupium very distinct, occupying the swollen part of the 

 shell. 



516 



