SCAMMON : THE UNIONID^ OF KANSAS, PART I. 311 



neither compressed nor inflated. Anterior margin variably 

 curved ; ventral margin decidedly bowed ; posterior margin 

 roundly or sharply pointed ; dorsal margin straight, slightly 

 oblique, and meeting the upper part of the posterior margin 

 at an angle of 150 degrees. Umboidal ratio, 0.30. Um- 

 bones rather full, not high, curved inward and downward, 

 with numerous fine concentric ridges, which are some- 

 times double-looped. Anterior umboidal slope rather flatly 

 rounded ; posterior slope sharp near the umbones, but rounded 

 ventrally, slightly excavated. Epidermis smooth, some- 

 times shiny, marked with numerous fine, fairly continuous 

 lines of growth, in color variable ; type A, dirty brown with 

 a greenish cast, marked with numerous dark rays more or 

 less green, conspicuous; type B, light horn, with brown 

 rays made up of V-shaped dashes of dark color directed 

 ventrally ; all variations occur between these two ; color over 

 umbones generally a lighter shade. Ligament short, thick, 

 dark brown. 



Interior : Pseudocardinals double in the left and single in 

 the right valve, high, thin, pointed, the left posterior de- 

 cidedly curved dorsally. Laterals straight, long, directed 

 ventrally, the dorsal lateral in the left valve often degen- 

 erate. Anterior adductor cicatrix much longer than wide, 

 well excavated dorsally; posterior cicatrices small, lightly 

 rarely confluent. Pallial line slightly impressed anteriorly. 

 Dorsal muscle scars often large, in the cavity of the beaks. 

 Cavity of beaks deep, of shell moderate. Nacre white, irides- 

 cent posteriorly. 



Dorsal 

 Length. Height. Breadth. Una. ra. posterior 



angle. 



63 33 22 0.28 144° 9 (400.1) 



45 22 18 .30 155° c? (401.1) 



52 30 21.5 .30 150° cJ" (400.2) 



38 25 15 .32 157° 9 (402.1) 



P. donaciformis ranges from Michigan south to the Ala- 

 bama river and to Trinity river, Texas. It is common to all 

 the Kansas drainage systems and abundant in all of them. 

 It does not frequent the smaller streams but seems to be con- 

 fined to the sandy and muddy beds of rivers. Formerly it 

 was quite common in the eastern portion of the Kansas river. 



