Euphorbia (Subgen. Chamaesyce Sect. Anisophyllum) jaegeri, a Shrubby New Species from the Deserts of California, United States

Euphorbia jaegeri (Euphorbiaceae), an endemic to southeastern California, United States, is described as new and illustrated with photographs. It is known from two general locations, one in the Orocopia Mountains (Riverside County) and the other in the Marble Mountains and adjacent Bristol Mountains (San Bernardino County). The habitat is desert scrub on rocky hillsides and along arroyos, primarily in rock crevices or gravelly soils, at elevations from approximately 600 to 850 m. The new species belongs to Euphorbia subgen. Chamaesyce sect. Anisophyllum. It is distinguished by the combination of a shrubby habit and involucral appendages that are deeply parted into triangular to subulate segments. The exact affinities of the new species are not obvious, but it is compared with E. polycarpa and E. setiloba. With only four known occurrences, a fragmented distribution, and significant existing and potential threats to most of the populations, E. jaegeri is of high conservation concern.