SCAMMON : THE UNIONID^ OF KANSAS, PART I. 283 



Family UNIONID^. 



"Shell nacreous, with a thick epidermis, beaks usually 

 sculptured, often showing the remains of nuclear shell ; liga- 

 ment opisthodetic ; hinge with or without teeth, though with 

 vestiges of them in every genus ; when present, schizodont, 

 and arranged as pseudocardinals and laterals ; pallial line 

 usually simple ; prismatic border ordinarily narrow. 



"Animal with labial palpi 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 outer or inner or all four leaves of the branchise." 

 (Simpson.) 



HETEROGEN^. 



"Male and female shells different. The latter are inflated 

 in the post-basal region ; embryos are contained in the ovisacs 

 separated by a sulcus and occupying the hinder part of the 

 outer gills." (Simpson.) 



Genus TRUNCILLA Rafinesque, 1819. 



"Shell rounded or oval, solid, inflated, generally smooth 

 and rayed with a delicate beak sculpture which has a tend- 

 ency to be doubly looped, that in the female having a de- 

 cided inflation in the post-basal region, which is thinner than 

 the rest of the shell, of different texture, often toothed, and 

 usually radially sculptured ; laterals double in each valve, 

 the inner in the right valve smaller. Animal generally hav- 

 ing the inner gill united to the abdominal sac ; female with 

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

 shell and which has an inner ridge inside at some distance 

 above the edge ; marsupium very distinct, occupying the 

 swollen part of the shell." (Simpson.) 



Truncilla triquetra Rafinesque. Plate LXIII, fig. 1. 



Truncilla triquetra Rafinesque, Ann. Gen. Sci. Phys. Brussels, xni, 



1820, p. 300, pi. Lxxxi, figs. 1-4. 

 Unio triangularis Barnes, Amer. Jour. Sci., iv, 1823, p. 272, pi. xiii, 



fig. 17. 

 Unio formosus Lea, Trans. Amer. Phil. Soc, iv, 1834, p. Ill, pi. xiv, 



fig. 41. 



