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Within the genus Capnia, the Capnia californica species group is demonstrated to be monophyletic and limited in zoogeographic distribution to western North America. A new species, Capnia kersti, is described. It is remarkable in coming from central Oregon, well north of the known range of all other closely related members of the group. Characteristics for diagnosing the new species from all others in the genus are given. A revised key to the C. californica group, key illustrations, and distribution maps for all species in the group are given. Fully resolved phylogenies of the group based on a matrix of morphological characters are given. The distribution of characters on the most parsimonious tree is discussed. Two types of distribution for members of the group are summarized, one along the coast of western North America and a second that is expanded to include the desert southwest. A large gap between closely related forms is noted in the region of the current Mohave Desert. This gap correlates to a date of between 8 and 15 ka (ka is 1,000 yr before present) for the desertification of this area.