Plants green with purplish undulate margins and deep purple ventral side; branching dichotomous or thalli simple; intercalary lateral branches rare. Thalli 5 10 ´ 2 4 mm; epidermis smooth, cells mostly 30 x 25 µm, thin walled, with very small trigones, oil cells absent; air pores surrounded by 1 2 tiers of 5 6 cells with slightly thickened radial walls; ventral scales imbricate, deep purple with scattered oil cells; appendages 1 2, purplish or hyaline, subulate to acuminate, often with few small marginal teeth, extending slightly beyond thallus margins at apices. Sexual condition paroecious, occasionally autoecious; androecia small dorsal group of papillae at posterior base of gynoecial stalks or occasionally forming mid dorsal streaks of papillae; gynoecia terminal on principal thalli; stalks naked, without hairlike scales at bases or apices, 1 2 cm, purplish; carpocephala high domed to ovoid or conical, about 4 mm high, 2.5 mm wide, with 3 4 lobes directed downward; pseudoperianths white, conelike, cleft 1/3 1/2 the length into 8 10 or more segments with attached apices. Sporophyte capsules dark brown; spores dark brown, 60 80 µm; elaters curved, brown, 2 spiral, 150 180 µm.

Capsules mature Feb.--Apr. Soil banks in rather dry situations; sw Calif.; Mexico (Baja California Norte).

This species can be recognized when fruiting by the dark brown spores. Other species have yellow or purple spores except A. echinella which has pale brown spores and is unlikely to be confused with A. palmeri because of the strongly tuberculate carpocephala of the former.