SCAMMON : THE UNIONID.E OF KANSAS, PART I. 335 



in size, long, somewhat fused, impressed, often deeply. Pal- 

 lial line impressed anteriorly and often its entire length. 

 Dorsal cicatrices a row of pits on the under side of the inter- 

 dentum. Hardly any umboidal cavity ; cavity of shell moder- 

 ate, branchial outline distinct. Nacre from milk-white to 

 pink or deep purple, sometimes almost leaden, often white 

 anteriorly and purple posteriorly ; iridescence hardly present. 



U. gibbosns is distributed throughout the Mississippi drain- 

 age, the St. Lawrence drainage, and the Alabama drainage. 

 It is also found from northern Florida west to the Guadalupe 

 river, Texas. Its distribution in Kansas is rather peculiar. 

 In the clear waters of the southern drainage it is a common 

 species ; in the Marais des Cygnes and its tributaries it is 

 very abundant, perhaps the most so of any Unto. In the 

 Kansas drainage, however, I know of only one specimen 

 being found. This was a single valve, picked up at Blanton 

 ford, on the Wakarusa river, near Lawrence. This is the 

 only specimen from this stream, although the river has been 

 thoroughly collected. It is found both on gravel bars and 

 riffles, and in deep water, embedded in the mud. 



Certain characters of the shell of this species are extremely 

 variable. Perhaps the col6r of the nacre is the most striking 

 of these. It is possible to make up a complete series of 

 specimens of this species, showing a gradual variation of 

 nacre from a deep leaden purple to pure white. Of the white 

 forms the most common is that with a tinge of purple dorsal- 

 anteriorly. Occasionally specimens are found in which the 

 cavity of the shell is pink. In my experience the purple 

 nacre form is the most common in the Neosho river, while 

 the white nacre form is the most abundant one in the Marais 

 des Cygnes. The thickness of the shell is quite variable. 

 There is also much variation in the length and curvature of 

 the dorsal margin ; in some specimens the extreme curvature 

 and length of this margin give the shell a peculiar humped 



