30 BOTANICAL GAZETTE [JULY 
Euphorbia robusta society——This society occurs on most of the 
hilltops and over rather large areas of the north Mesa. E. robusta 
is less dominant than the controlling species of the societies already 
mentioned. Echinocereus viridiflorus is in point of numbers more 
abundant but it is comparatively a very inconspicuous plant. Sen- 
ecto oblanceolatus is not dominant in this society, but ranks second 
in importance to Euphorbia robusta; the society very naturally 
Fic. 7.—Typical Mesa: Bouteloua formation; Yucca glauca society. 
grading into the Senecio oblanceolatus society almost imperceptibly. 
Secondary species are Lesquerella montana, Vucca glauca, Oreocaryd 
thyrsiflora, Aragallus Lambertii, Mertensia linearis, and Erigeron 
pumilus. The soil is a coarse, gravelly loam; water content 6-9 per 
cent. 
Arenaria Fendleri society.—On crests in coarse gravel or gravelly 
loam, where the water content is low, this society is found. A: 
Fendleri is dominant, but many other species help to make up 
bes 
