Occasional Papers of the Museinn of Zoology 29 



that of A. cxpUcata (Mo.). If this arrangement is correct, 

 L. roz'irosai Pilsbry shows a much more marked sexual dimor- 

 phism than does the latter species, where the female is nearly 

 the same shape as the male but is slightly more inflated along 

 the posterior ridge and down to the ventral margin. This more 

 marked dimorphism and the indications of the beak sculpture 

 (even the younger specimens are somewhat eroded) are the 

 reasons for the retention of rovirosai in Lampsilis, until the 

 soft parts are known. 



Measurements 



bi) 



c 





D 



Lampsilis ruthveni, new species 

 Plate XI, fig. 53; plate XII, f^g. 53; plate XIII, figs. 51-54 



Seven specimens, including odd valves (2), from the Rio 

 San Juan (H, vii, c). 



Shell rather small, elliptic to subovate, rather solid, inflated 

 behind middle; beaks in front of middle low; beak-sculpture 

 not observed ; posterior ridge rounded or shouldered ; posterior 

 margin with rounded point at or a little below middle of height ; 

 epidermis smoothish, thrown into sulcations and low, rounded 

 ridges at growth-lines, dull, golden-brown to dark brown, the 



