310 KANSAS UNIVERSITY SCIENCE BULLETIN. 



pointed, ragged, double in the left and single in the right 

 valve. Laterals rather long, thin, curved. Anterior adductor 

 cicatrix an elongated quadrant, well impressed dorso-poste- 

 riorly ; posterior cicatrices large, lightly impressed, confluent. 

 Dorsal scars rather large pits in the cavity of the umbones. 

 Pallial line well impressed anteriorly. Cavity of beaks and 

 of the shell considerable. Nacre white, quite iridescent. 



This beautiful little Unio has a general range coextensive 

 with that of the preceding member of this genus. Its range 

 in Kansas, however, far exceeds that of P. securis, for it is 

 common to all the drainage systems of the state. It has been 

 reported as far west as Wildcat creek, Riley county (Popenoe) 

 in the Kansas drainage, and is found as far west as the Little 

 Arkansas in the southern drainage (Call). Its habitat is the 

 muddy or sandy beds of streams of some size. In ray collect- 

 ing I have never found this form abundant, although present 

 in all good-sized streams. It is often found in the Wakarusa 

 and sometimes in the Kansas river at Lawrence. 



The variability of this form is slight and is confined mainly 

 to the color and character of the epidermis. The umboidal 

 ridge is much more prominent in young than in old shells. 

 Its nearest relation, P. donaciformis , can be easily identified 

 from elegans by the greater length and the finer pseudocar- 

 dinals of the former. The relation of the height to the length 

 of the shell in elegans averages 1 : 1.3. The same relation in 

 donaciformis is 1 : 1.8. P. elegans can easily be separated 

 from the young of the trigonal Quadrulas by its thinner shell 

 and sharp, erect teeth. 



Plagiola donaciformis Lea. Plate LXX, figs. 1, 2. 



Unio donaciformis Lea, Trans. Amer. Phil. Soc, ni, 1827, p. 267, pi. 



IV, fig. 3. 

 Unio zig-zag Lea, Trans. Amer. Phil. Soc, ni, 1829, p. 440, pi. xil, 



fig. 19. 



Shell small, moderate in thickness, elongate ellipsoid, 



