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The Western Lyresnake (Trimorphodon biscutatus) inhabits arid regions from the desert southwestern United States southward along the Pacific lowland versant to northwestern Costa Rica and exhibits substantial geographic variation in size, squamation, and color pattern across its range. We examined patterns of geographic variation within T. biscutatus using multivariate statistical analyses of 33 morphological characters scored from 429 specimens. Principal components and discriminant analysis revealed six morphologically distinct groups that are generally concordant with lineages recovered in a phylogeographic analysis of mitochondrial DNA and with taxa traditionally recognized as species or subspecies. We conclude that Trimorphodon biscutatus (sensu lato) comprises six evolutionary species (including the recently elevated T. vilkinsonii) and recommend elevating T. biscutatus (sensu stricto), T. lambda, T. lyrophanes, T. paucimaculatus, and T. quadruplex to the species level. A key to the species of Trimorphodon is provide. [ABSTRACT FROM AUTHOR]