546 RyDBERG: Rocky MOUNTAIN FLORA 
CARDUUS AMERICANUS XSPATHULATUS 
This resembles most C. americanus in habit and leaf-form; the 
bracts are somewhat erose on the margins as in that species, but 
they are scarcely at all dilated, and if so only the innermost, and 
they are tipped with the short and broad spines characteristic of 
C. spathulatus. To this are referred: 
Co.Lorapo: Estes Park, Aug. 16, 1905, Osterhout 3091; Sulphur 
Springs, July 16, 1905, Osterhout 3057; Happy Hollow, July 14, 
1898 (collector not given), Herb. State Agric. College, mo. 2801. 
The last was distributed as Carduus griseus and has perhaps 
given rise to a wrong impression of that species. C. spathulatus 
was then undescribed and the bracts excluded no. 2801 from C. 
americanus. We have no specimens of either of the supposed 
parents, from exactly the same locality, but C. americanus is found 
nearly everywhere in the mountains of northern Colorado, and 
Osterhout in the original description of C. spathulatus states that 
it is common on both sides of the range of mountains east of the 
North Park. 
CARDUUS AMERICANUS X COLORADENSIS 
Carduus erosus Rydb. Bull. Torrey Club 28: 507. Igot. 
This was originally described as a distinct species. Professor 
Nelson reduces it to a synonym of Carduus americanus. The 
broad hemispheric head, the broad bracts with less dilated tips, 
and the more spiny leaves with more numerous and lanceolate 
lobes are very different from those of the typical C. americanus. 
The form and structure of the involucre, the form of the leaves, 
and the habit approach those of C. coloradensis. The upper 
surface of the leaves and the midrib beneath show some of the 
arachnoid hairs characteristic of C. coloradensis and its allies. 
We have no specimens of the two supposed parents from Durango, 
the type locality of C. erosus, but the locality is not without the 
range of either. 
CoLorAbDo: Durango, 1896, F. Tweedy 517. 
CARDUUS ACAULESCENS X AMERICANUS 
This resembles most C. americanus, but the stem is lower, 
the heads crowded, the involucral bracts elongated and less dilated 
