Cryptantha whippleae D.A.York & M.G.Simpson (Boraginaceae) is described as new. This species is currently known to occur in serpentine barrens in the Shasta-Trinity National Forest of Siskiyou County, California, with one outlier population in possible serpentine of Lake County, California. The new species is most similar to Cryptantha grandiflora and to C. milobakeri, these three likely each others’ closest relatives. All three have a relatively large corolla limb width and similar smooth, lance-ovate to ovate, marginally rounded, acuminate and abaxially transversely flattened nutlets. Cryptantha whippleae differs from C. grandiflora in having a short, as opposed to a tall, stem height; bifurcate as opposed to trifurcate primary axis cymules; and typically 2–3 nutlets per fruit, as opposed to usually one nutlet per fruit. Cryptantha whippleae differs from C. milobakeri also in having a short, versus tall, stem height; appressed-strigose and spreading-hispid stem vestiture, as opposed to strigose only or strigose and hirsute; calyx trichomes with two distinct vestiture types, these marginally appressed hirsute and medially hispid, as opposed to calyx trichomes of one type, dense, appressed to ascending, whitish sericeous; and 2–3 nutlets per fruit, as opposed to one nutlet per fruit. Cryptantha whippleae is relatively rare and joins seven other Cryptantha species that are found on serpentine, either obligately or facultatively. Current molecular phylogenetic studies support the mostly convergent evolution of serpentine adaptation in Cryptantha, but additional studies are needed.

Key words: Boraginaceae, California, conservation, Cryptantha whippleae, endemic, Klamath Mountains, serpentine, taxonomy

 Cryptantha whippleae D.A.York & M.G.Simpson, sp. nov.

Diagnosis: Cryptantha whippleae is similar to C. grandiflora in having a relatively wide corolla limb [3–6 mm wide in C. whippleae; 4–8 mm wide in C. grandiflora] and in the size, shape and sculpturing of nutlets, differing in having a mostly shorter plant height [3–8(15) cm tall in C. whippleae versus 5–35 cm tall in C. grandiflora], in cymule branching [bifurcate or rarely solitary in C. whippleae versus trifurcate in C. grandiflora] and in having more nutlets per fruit [2–3 in C. whippleae versus (2) in C. grandiflora]. Cryptantha whippleae is similar to C. milobakeri in corolla limb width [3–6 mm wide in C. whippleae; 2–6 mm wide in C. milobakeri] and in the size, shape and sculpturing of nutlets, differing in having a shorter height [3–8(15) cm tall in C. whippleae versus 10–50 cm tall in C. milobakeri], in calyx vestiture [with two distinct trichome types in C. whippleae versus a single trichome type in C. milobakeri] and in having more nutlets per fruit [2–3 in C. whippleae versus 1(2) in C. milobakeri].