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This systematic study documents the taxonomy, diversity, and distribution of 40 species of the predominately Californian trapdoor spider genus Aptostichus Simon, 1891. Thirty-three of these species are newly described: A. dantrippi, A. cabrillo, A. pennjillettei, A. asmodaeus, A. nateevansi, A. chiricahua, A. icenoglei, A. isabella, A. muiri, A. barackobamai, A. sinnombre, A. hedinorum, A. aguacaliente, A. chemehuevi, A. sarlacc, A. derhamgiulianii, A. anzaborrego, A. serrano, A. mikeradtkei, A. edwardabbeyi, A. killerdana, A. cahuilla, A. satleri, A. elisabethae, A. fornax, A. lucerne, A. fisheri, A. bonoi, A. cajalco, A. sierra, A. huntington, A. dorothealangeae, and A. chavezi. Most of these species are restricted to the California Floristic Province, a known biodiversity hotspot. Of the 40 recognized species, over half are considered to be imperiled or vulnerable and two have likely gone extinct over the past half-century; the conservation status of only 11 species is considered to be secure. Using 73 quantitative and qualitative morphological characters I propose a preliminary phylogeny for the genus that recognizes four major lineages: the Atomarius, Simus, Hesperus, and Sierra species groups. Additionally, the phylogenetic analysis indicates that adaptations favoring the invasion of the arid desert habitats of southern California have evolved multiple times across the group. The existence of both desert and non - desert species in three of the four species groups makes this genus an ideal candidate for the study of the evolutionary ecology of desert arthropods. A set of molecular characters based on the contiguous mitochondrial DNA genes 16S-tRNA valine-12S is used in an independent analysis to assist in placement of specimens into species. The taxonomy section explicitly identifies the concept employed in species delimitation. Niche based distribution models are constructed to predict the ranges of species for which an adequate number of sampling sites were known.