8o Bryant Walker 



Genus LEMIOX (Rafinesque ??j Ortmann, 1916. 



Shell triangularly ovate, solid, more or less inflated, with a low, rounded 

 posterior ridge; beaks high, turned forward over a small lunule, sculpture 

 ^Tf-^,. distinctly double-looped; surface with strong, cor- 

 . ^f^'^' - '■'^^ 4. rugated, subradial sculpture on the posterior half, 

 J^'^ ' .'^ - "% which is divaricate on the posterior ridge and which 



; '' sometimes covers the entire shell ; epidermis cloud- 



"^ ed, dull-green or yellowish green, usually feebly 



rayed ; pseudocardinals low, subradial, ragged, two 

 in the left valve, one to three in the right valve; 

 Fig. 216 laterals heavy, double in the left valve, partly dou- 



ble in the right ; muscle scars small, impressed ; nacre silvery white, irides- 

 cent behind, thicker in front. Male shell subovate or subtriangular, witli a 

 broad, shallow, radial depression in front of the posterior ridge or having 

 the whole disk, at least, flattened. Female shell usually ovate, sometimes 

 inflated, smaller than the male, with a feebly developed marsupial swelling, 

 distinctly, but irregularly, denticulate on the margin, at the base near the 

 posterior end. 



Mantle margin of the female in front of the branchial opening denticu- 

 late on the outer margin. The inner margin has, just in front of the l)ran- 

 chial, a few small papilLx and then is laminate and elevated and rather 

 smooth and probably capable of some expansion. 

 Type: Unio ccclatits Con. (Unio rimosiis Raf. ??), fig. 216. 



Genus TRUNCILLA Rafinesque, 1819. 



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

 and rayed; beak sculpture delicate, often obsolete, double-looped; female 

 shell very different from that of the male, having a very decided inflation 

 in the post basal region, which is thinner than the rest of the shell, of dififer- 

 ent texture, often toothed and usually radiately sculptured. 



Inner edge of the mantle in the female in front of the branchial open.ing 

 is not parallel to the outer edge, but is more or less remote from it. often 

 quite distant from it and has finer or coarser papillfe. The mantle between 

 the two edges is pecidiarly spongy. Marsupium swollen, kidney-shaped, 

 formed by many ovisacs, occupying the posterior portion of the outer gill. 



Key to the subgenera of Truticilla. 



r Male shell smooth, no radiate, posterior furrow ; female* 



1. i shell inflated along posterior ridge TnincUla r. .r. 



I Male shell with a wide, radiate, posterior furrow 2. 



r Female shell with a small, rounded, radial post-basal swell- 



I ing Scalcnar'ia. 



2. \ Female shell with a greatly produced basal swelling, nearlv 



! in the centre of the base Dysnoiuia. 



I Female .shell with a rounded, foliaceous swelling at the pos- 



[ terior base Pilca. 



