356 KANSAS UNIVERSITY SCIKNCE BULLETIN. 



Q. pustulosa ranges over the entire Mississippi drainage and 

 into Michigan, it extends east into the Alabama river system 

 and west to central Texas. It is quite common throughout 

 the Kansas drainage systems. It is found in streams of some 

 size, either in gravel or mud bottom. 



Q. jnistulosvs is one of the most variable species of an ex- 

 tremely variable group. It is hardly possible to write a 

 description of this form which will cover all its varied muta- 

 tions. The general outline may assume almost any form 

 except a decidedly elongate one, and the umboidal ratio has 

 absolutely no stability. The pustules may crowd the disk or 

 the shell may be practically smooth. The pseudocardinals 

 and laterals are subject to much variation both as to position 

 and to form, and the interdentum may be broad and long or 

 almost absent. 



The main distinctions between this and other nearly related 

 forms will be found under the latter. 



Quadrula pustulata Lea. Plate LXXXIII, fig. 2. 



Unio pustulatus Lea, Trans. Amer. Phil. Soc. , 1834, p. 79, pi. Vll, fig. 9. 

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



Shell rather small, solid, particularly so anteriorly, sub- 

 orbicular but for a square dorso-posterior projection, inflated. 

 Anterior margin and anterior half of the ventral margin circu- 

 larly rounded ; posterior half of the ventral margin straight 

 or slightly emarginate ; posterior margin straight or very 

 slightly incurved, meeting the dorsal margin at an approxi- 

 mate angle of ninety degrees ; dorsal margin straight, ob- 

 lique. Umboidal ratio, 0.20 to 1.35. Umbones very full, 

 fairly high, decurved, bearing three or four small concentric 

 ridges on their tips. Anterior umboidal slope fully rounded, 

 as is also the lateral one ; posterior slope abrupt anteriorly, 

 very flatly curved posteriorly, being produced dorso-posteriorly 

 into a sort of triangular wing, which, however, is in no way 



