RyDBERG: Rocky MOUNTAIN FLORA 161 
The species on which this genus is based was first described 
as a Prenanthes and later transferred to Lygodesmia. It is so 
unlike the other species of Lygodesmia in habit, that the habit 
alone would suggest a distinct genus. But the best character is 
found in the achenes, which are ribbed and tapering downward from 
the summit instead of upwards from the base. The fruit agrees 
better with that of adalus (Prenanthes), but from this genus Pren- 
anthella is distinguished by the habit and the structure of the 
involucre. 
Prenanthella exigua (A. Gray) Rydb. 
Prenanthes exigua A. Gray, Pl. Wright. 2: 105. 1853. 
Lygodesmia exigua A. Gray, Proc. Am. Acad. g: 217. 1874. 
New York BoraNnicaAL GARDEN. 
