552 Rydberg : Rocky Mountain flora 



with numerous short crowded lobes and densely white-tomentose 

 beneath. The spines of the involucral bracts are also long and 

 strong, in age usually strongly spreading. C. megacephaliis , 

 which resembles it in many respects, has much broader flatter 

 leaves, with fewer lobes and short spines, scarcely decurrent 

 and at least the upper ones with broad clasping bases. The spines 

 of the involucres are also short and weak. The following two 

 specimens have leaves resembling those of C. ochrocentrns but 

 not decurrent and have involucral bracts with the short weak 

 spines of C. megacephaliis. 



Nebraska: Banner County, July 6, 1891, Rydberg 215a. 



Kansas: Plains, Ellis County, July 16, 1895, Hitchcock jog. 



The first of these was collected with Cardmis megacephalus, 

 Rydberg 215. C. ochrocentrus was common in the region. Rydberg 

 214, belonging here, was collected a few miles further south. 

 There are also forms intermediate between C. megacephalus and C. 

 undiilatus, but as these two species are so closely related that it 

 is almost impossible to draw a line between them, I have not tried 

 to distinguish any hybrids. 



Carduus foliosusXscopulorum 

 This has the habit and bracts of Carduus foliosus, but the in- 

 volucre is densely arachnoid as in C. scopuloriim and the leaves 

 have more numerous and crowded lobes, in that respect approach- 

 ing those of the latter species. 



Wyoming: Big Horn Mountains, Aug., 1899, Tweedy 2120. 



Carduus scopuloriim was evidently growing near it, for a 



specimen belonging to it and collected by Tweedy bears the 



number 2122. C. foliosus is common in the Big Horn Mountains. 



Among others are Tweedy 305 1 , collected there the following year. 



Carduus Tweedyi, judging from the scarcity of the plant and 



from the fact that it combines the characters of two groups, may 



also be a hybrid. A plant of its type may be produced by the 



crossing of C. polyphyllus and a red- flowered species such as 



C. Macounii or C. ediilis but neither of these two has been found 



east of the continental divide and C. Tweedyi not west of it. A 



somewhat similar plant would be produced by the crossing of 



C. scopulorum and C. Eatoni, but I have seen no specimens of the 



latter outside of Utah. 



