Subacaulescent, densely tufted or mounded, the stems of the year reduced to leafy crowns elevated year by year on branches of a closely and repeatedly forking caudex beset with a thatch of marcescent stipules and recurving petioles, together forming low cushions or domes of foliage up to 5 dm. in diameter (but mostly less), silvery-strigose with straight, appressed or narrowly ascending, lustrous hairs up to 1—1.4 mm. long, the herbage canescent, the stipules glabrous dorsally, lustrously hyaline; stipules 4-8 mm. long, united behind the petiole and (at least in vernation) connate opposite it into a sheath; leaves 1-4 (5) cm. long, slenderly petioled, the leaflets linear-oblanceolate, oblanceolate, or in some short leaves produced in early spring elliptic-obovate, acute or subacute, mostly involute or conduplicate, 3-12 mm. long; peduncles slender, 7-22 mm. long, at anthesis erect, in fruit recurved-spreading or the outermost on a plant prostrate and radiating; racemes loosely (1) 2-4-flowered, the flowers ascending, the axis up to 0.5—1.5 cm. long in fruit; bracts scarious, lanceolate, linear-lanceolate, or subsetaceous, 1.8-3.5 mm. long; pedicels ascending, straight or nearly so, 0.7-1.5 mm. long, scarcely thickened or elongating in fruit; calyx (4.8) 5—7.1 mm. long, densely white-strigose, the deeply campanulate or short-cylindric tube 3.6-5.1 mm. long, 1.8-2.4 (3) mm. in diameter, the subulate or linear-subulate teeth (1.4) 1.9-5.1 mm. long, the whole becoming papery, marcescent unruptured; petals pink-purple with a pale, striate lozenge in the fold of the banner; banner recurved through ± 40°, obovate beyond the long-cuneate claw, or oblanceolate, shallowly notched, (9.6) 10.5-16.7 mm. long, (3.8) 4.4—8.3 mm. wide; wings 9.1-13.5 mm. long, the claws 4.1-5.5 mm., the linear-oblong, obtuse, or obliquely obovate and subemarginate, slightly incurved blades (4.6) 5.3-8.5 mm. long, 2.3-3.1 mm. wide; keel 7.5-10.5 mm. long, the claws 4.3-6 mm., the lunately half-elliptic blades 3.5-4.8 mm. long, 1.6-2.5 mm. wide, incurved through 60-85° to the acutely triangular, often subporrect apex; anthers (0.45) 0.5-0.75 mm. long; pod ascending, sessile, narrowly lance-ellipsoid, 4.5-7.5 mm. long, 1.2-2.2 mm. in diameter, tapering distally into a slightly incurved, lance-subulate beak nearly as long as the fertile part, when fully ripe obscurely trigonous in the lower half, carinate ventrally by the suture, a little flattened dorsally, the thin, silvery-strigulose valves becoming papery, not inflexed; dehiscence basal and upward through both (but primarily through the ventral) sutures; seeds (commonly only 1 maturing) oblong-cylindroid, not or scarcely compressed, dull greenish-purple, smooth but dull, 1.9—2.1 mm. long.—Collections: 11 (i); representative: Tweedy 3156 (NY, RM); Barr (from 3 localities in Fall River and Shannon Counties, South Dakota) in 1952 (RSA), 2011, 2012, 2013 (RM); Barneby 13,231 (CAS, GH, NY, RM, RSA, US); Schunk & Schwantz (from Ekalaka, Montana) in 1943 (UTC).
Gullied knolls, buttes and barren hilltops, mostly below 4700 feet, on limestone or sandstone, apparently uncommon but locally plentiful, known only from the valley of the White River in southwestern South Dakota and from the Powder River drainage in northeastern Wyoming and extreme southeastern Montana. Map No. 162.—May to June, sometimes in late April in years of light snowfall.
Astragalus Barrii (Claude Amo Barr, 1887— , proprietor of Prairie Gem Ranch, nursery near Oelrichs, South Dakota) Barneby in Amer. Midl. Nat. 55: 506. 1956. South Dakota. Limestone Butte (SE of Oelrichs), Fall River Co., May 4, 1952, Claude A. Barr. Holotypus, CAS!
The Barr orophaca is easily distinguished from the other Sericoleuci by the length of the petals and by the moderate curvature of the banner and of the triangular, not bluntly rounded blades of the keel. The allopatric A. tridactylicus alone approaches it in flower-size, but this is ordinarily a simply tufted or matted plant never elevated into a domed cushion, and it differs technically in its dorsally pubescent stipules. The dispersal of the Barr orophaca is still imperfectly known, but it inhabits a tract of prairie country which has been neglected by botanists and from which there are few astragalus records of any kind. In South Dakota it is confined to a limestone caprock overlying the Pierre formation and, according to Mr. Barr, is an insular relic in this region. Near Biddle, Montana, the species is found on sandstone and no reason for its extreme localization there is apparent. Nothing is known of its habitat in Wyoming. Compared with other Sericoleuci, which seem to fruit very sparingly and in some years not at all, A. Barrii is prolific with fertile pods, produced even under cultivation in the East.