SCAMMON : THE UNIONID^ OF KANSAS, PART I. 



309 



pressed shell, with a comparatively weak musculature. A 

 comparison of forms from these two different habitats in the 

 same stream are given below : 



*This ratio is taken from male shells in both cases. 



A complete series of gradations may be found between these 

 two extremes. The dimorphism of this form is indicated by 

 the somewhat greater inflation of the female. There is no 

 other Kansas Unio with which this can easily be confused. 



Plagiola elegans Lea. Plate LXX, fig. 3. 



Unio elegans Lea, Trans. Amer. Phil. Soc, iv, 1831, p. 83, pi. ix, fig. 13. 

 Unio truncatis Say, Amer. Conch., vi, 1834. 



Shell rather small, somewhat thickened anteriorly, thin 

 posteriorly, trigonal, inflated. Anterior margin curved; ven- 

 tral margin curved, often somewhat emarginate the posterior 

 third ; posterior margin pointed, sometimes straight and join- 

 ing the dorsal margin at a wide angle and sometimes form- 

 ing a continuous gentle curve with it ; dorsal margin short, 

 straight or gently curved oblique. Umboidal ratio, 0.25 to 

 0.30. Umbones full, elevated, slightly incurved, marked in 

 some cases with a few faint concentric ridges. Anterior 

 umboidal slope fully rounded ; lateral slope fully rounded 

 anteriorly and marked by a broad furrow in some cases ; pos- 

 terior umboidal ridge sharp, well defined, and continuous 

 from umbones to margin ; posterior slope slightly incurved. 

 Color of the epidermis from grass-green to dark brown, often 

 rayed; rays thin, thread-like, sometimes wavy, crowded. 

 Lines of growth fairly continuous, even, rarely imbricated. 

 Ligament short, dark brown. 



Interior : Pseudocardinals erect, very high and sharp 



S-Bull., Vol. Ill, No. 9. 



