334 KANSAS UNIVERSITY SCIENCE BULLETIN. 



lateral in the right, with rarel} 7 a vestige of a second lateral ; 

 cavity of the beaks not deep or compressed. 



"Animal having the inner branchiae free from the abdomi- 

 nal sac for from one-half to their entire length ; marsupium 

 occupying the whole length of the outer gills only, forming 

 a thick, smooth bag when rilled with young; gills united to 

 the mantle behind to their extreme points or very nearly 

 so ; papillae on branchial and anal openings unbranched ; su- 

 peranal opening always closed below." (Simpson.) 



Unio gibbosus Barnes. PL LXXVII. 



Unio gibbosus Barnes, Amer. Jour. Sci. and Arts, xiv, 1828, p. 286. 

 Unio arctior Lea, Trans. Amer. Phil. Soc. , vi, 1838, p. 10, pi. iv, fig. 10. 



Shell rather large, thick, but variable in this respect, 

 thickest anteriorly, elongate, flat ellipsoid, often tapering 

 posteriorly, compressed. Anterior margin shortly rounded; 

 ventral margin straight, often slightly sulcate posteriorly ; 

 posterior margin more or less roundly pointed ; dorsal mar- 

 gin long and curved, sometimes curving insensibly into the 

 posterior margin, oblique, and set on an average angle of 

 about twenty-five degrees from the ventral margin. Urn- 

 boidal ratio quite constant, approximately one-fourth of the 

 entire length. Umbones fiat, and marked with several coarse 

 concentric ridges. Anterior uniboidal slope rounded; lateral 

 slope llatly rounded and sometimes marked with a broad, 

 shallow radial furrow posteriorly ; posterior uniboidal slope 

 abruptly curved ; umboidal ridge quite well marked. Lines 

 of growth numerous, continuous, in older specimens imbri- 

 cated, particularly posteriorly. Ligament long, stout, rich 

 brown in color. Lunule long and slender. 



Interior : Pseudocardinals low, rounded, thick, evenly ser- 

 rate, double in the left and single in the right, but often with 

 one or two auxiliary spurs in the right valve. Laterals long, 

 more or less oblique, straight, slightly curved, slightly 

 roughened posteriorly. Interdentum broad and long. An- 

 terior adductor cicatrix of moderate depth and size, some- 

 times placed slightly under the anterior pseudocardinals, 

 particularly in old specimens. Retractor cicatrix well im- 

 pressed, variable in shape and size. Posterior scars moderate 



