Low, slender, caulescent perennial, with taproot and subterranean root-crown, strigulose throughout with fine, appressed and subappressed hairs up to 0.2-0.3 (0.4) mm. long, the herbage greenish-cinereous or cinereous, the leaflets glabrous above; stems 2-26 cm. long, decumbent and incurved-ascending, buried and leafless for 1-10 cm., commonly branched on emergence, simple thereafter or shortly spurred at 1-3 nodes preceding the first peduncle; stipules 1.5-5 mm. long, the lowest papery-scarious or early becoming so, the upper ones subherbaceous, triangular-ovate to lance-acuminate, about semiamplexicaul; leaves 2.5-7 (9) cm. long, all but the lowest subsessile, with (11) 13-23 linear-oblong to narrowly oblanceolate, obtuse or retuse, mostly folded leaflets (2) 4-12 (14) mm. long; peduncles erect or incurved-ascending, 3.5-9 cm. long, as long or commonly longer than the leaf; racemes densely, or in age somewhat loosely 15-37-flowered, the flowers early reflexed and retrorsely imbricated, the axis somewhat elongating, (1.5) 2-7 (8) cm. long in fruit; long.—Collections: 8 (o); representative: Pringle 6404 (K, ND, NY, US), 9722 (K, NY, US); Rose & Painter 7090 (NY, US); Matuda 21,222 (MICH, NY).
Dry stony fields, grassy knolls, and open hillsides, often on limestone but probably also on volcanic bedrock, 6500-9300 feet, apparently not uncommon in the valley of Mexico (State and Federal District) and southern Hidalgo.— Map No. 155.—June to September.
Astragalus oxyrrhynchus (sharp-beaked, of the pod) Hemsl., Biol. Centr.-Amer., Bot. 1: 265. 1880.—"South Mexico, Tizapan, valley of Mexico (Bourgeau 329)"; Diagn. Pl. Nov. Mex. 42. 1880.—"In convalli Mexici, Bourgeau."—Holotypus, Bourgeau 329, collected June 18, 1865, K! isotypi, labeled "Pâturages à Zapan, pres Mexico," P, US!—Tragacantha oxyrrhyncha (Hemsl.) O. Kze., Rev. Gen. 947. 1891. Hesperastragalus oxyrrhynchus (Hemsl.) Rydb. in Bull. Torr. Club 53: 165. 1926.
Astragalus angelinus (of San Angel) Jones, Rev. Astrag. 286, Pl. 71. 1923 ("Angelinus"). —"From near San Angel valley of Mexico, J. N. Rose, Aug. 21, 1903. Nat. Herb. No. 450040 ... "—Holotypus (Rose & Painter 6490), US! isotypus, NY!
The sharp-beaked milk-vetch, A. oxyrrhynchus, differs from other Micranthi in its buried root-crown, dwarf stature, and pod of plumper outline not or only obscurely sulcate dorsally. The very small, abruptly reflexed flowers are typical of the section and the fruit differs only in its less pronounced trigonous compression. There is no reason to assume on the basis of this last feature that A. oxyrrhynchus is related to sect. Didymocarpi or the genus Hesperastragalus, of which the genuine members (= our sect. Microlobium) are Californian annuals with tiny, globose, two-seeded pods. The position of the root-crown and general habit of growth are likely to suggest some small-flowered Strigulosi, but these have the lower stipules connate and pods dehiscent on the raceme. Under A. Hartwegi I have suggested, however, that A. oxyrrhynchus may have originated as a recombination type following introgression between forebears of sects. Strigulosi and Micranthi. The species also resembles A. hypoleucus in many ways, the latter, however, being easily distinguished in practice by its sheathing stipules and dolabriform vesture.
The pod of A. oxyrrhynchus is somewhat variable in size and outline, but I agree with Rydberg that the specimens singled out by Jones as A. angelinus are variants of a minor, inconsequential nature. Jones (1923) referred A. oxyrrhynchus to sect. Micranthi and A. angelinus to sect. Didymocarpi, implying fundamental differences. However his figures of the fruits (Pl. 70, 71) depict that of A. oxyrrhynchus as semibilocular and deeply sulcate dorsally, and that of A. angelinus as more or less didymous; in reality the cross-section in both is ovate-triangular, little or not at all notched dorsally, and the septum in both is wide and complete. Rydberg (1926, l.c.) associated Hesperastragalus oxyrrhynchus with H. reflexus, a Texan annual which has similarly formed pods but differs greatly in shape and proportions of the petals, in vesture, and also in other respects; the relationship is exceedingly remote.