Technically similar to A. sericoleucus, but pulvinate, the very short stems of the year and the young foliage elevated on repeatedly and closely forking, columnar caudex-branches to form mounded cushions 1-3 dm. in diameter, the herbage commonly more densely silky, satiny-lustrous when young, the longest hairs scarcely over 2 mm. long; stems of the year almost 0 or up to 1.5 cm. long, the internodes mostly concealed by dorsally glabrous (or subglabrous), ciliate stipules; leaves crowded, 1-2 cm. long, the leaflets 3-6 mm. long; peduncles ascending, 7-15 mm. long; racemes 2-flowered; calyx 3.3-4.2 mm. long, the tube 2.1-2.3 mm., the teeth 1.2-2 mm. long; petals brilliant pink- or magenta-purple to blue- or reddish- violet, rarely white; banner (5.8) 6.6-8 mm. long, 4-5.3 mm. wide; wings (5.7) 6-6.8 mm. long, the claws 1.7-2.1 mm., the blades 4.3-5.1 mm. long, 1.4—2.1 mm. wide; keel 4.1-4.5 mm. long, the claws 1.8-2.2 mm., the blades 2.5-2.9 mm. long, 1.4—1.7 mm. wide; anthers 0.3—0.4 mm. long; pod ± 4—4.5 mm, long, not seen fully formed, apparently similar to that of the preceding.—Collections: 11 (iii); representative: A. Nelson 4767 (NY, RM); Merrill & Wilcox 613 (NY, RM); Ripley & Barneby 7977 (CAS, RSA), 8920 (CAS, RSA, UTC); E. & L. Payson 2601 (GH, MO, RM).
Barren clay bluffs and eroded banks, on sandstone or limestone, 4000-5800 feet, locally plentiful along the east foothills of the Rocky Mountains in Wyoming, especially along the Wind and Big Horn Rivers, extending north along the latter stream just into Big Horn County, Montana, and south to Bitter Creek in the Red Desert (west of the Continental Divide); also in scattered stations eastward to the North Platte in Carbon County and to Sand Creek in Albany County.—Map No. 162.—June to August.
Astragalus aretioides (Jones) Barneby in Amer. Midl. Nat. 55: 505. 1956, based on A. sericoleucus var. aretioides (in habit resembling an Aretian Androsace) Jones, Contrib. West. Bot. 8: 13. 1898.—"H. Engelmann, Simpson’s Exp. Aug. 19, 1858, mountains near Sweetwater River, Wyo. This is the type."—Holotypus, MO!—Orophaca aretioides (Jones) Rydb. in Bull. Torr. Club 32: 655. 1905.
The cushion orophaca, A. aretioides, which differs technically from A. sericoleucus in its pulvinate growth-habit and lustrous, dorsally glabrous stipules, is by far the showier plant at anthesis. Although the flowers are only a trifle larger, they are substantially longer in proportion to the tiny, white-satiny leaflets; being more closely set together over the mounds of foliage, they make a telling effect when all are in bloom together. In some populations of A. aretioides along the upper Wind River, where the species is associated in places with the similarly mounded but entire-leaved A. simplicifolius, the flowers vary in color from a purple ordinary in the genus into deep bluish- and amethystine-violet and through successively paler shades of lilac and pinkish-lavender into a pure albino.
As mentioned under A. sericoleucus, the cushion orophaca was collected first by Nuttall, but not immediately recognized as distinct. Nuttall left no record of the locality, but this might well have been on the Sweetwater River where Engelmann encountered the species not long afterward. The var. aretioides described by Jones in his Revision (1923, p. 84) is not the original plant, but the matted phase of A. tridactylicus found on the red beds of the Laramie Plains. Likewise Rydberg’s O. aretioides, with "pubescent stipules" and a range extending into "northern Colorado," is based largely on the same misconception.