USNM (National Museum of Natural History, Smithsonian Institution) 7888; terra typica restricta: California, San Bernardino County; Mountains of California, near Fort Mojave; Soda Valley (very approximately 35°6'N, 116°6'W). We restrict the type locality to that published by Cooper (1861) and the parchment tag associated with the specimen. References to other localities, including Solado Valley, an apparent synonym of Soda Valley, should be considered to be in error.

The evolutionary species concept (Simpson 1961; Wiley 1978) suggests that the Sonoran population of the desert tortoise should be recognized as a new taxon. Frost and Hillis (1990) effectively argue that subspecies should not be recognized for continuously distributed species; we agree. Given these two observations, at least two species of desert tortoise should be recognized. The DNA sequence data exclude application of the available name Gopherus lepidocephalus for the Sonoran Desert population of Gopherus that occurs west and south of the Colorado River and they confirm that the lectotype of Gopherus agassizii is from the Mojave Desert, and not Arizona. Because no names are available for the tortoise population occurring in the Sonoran Desert south and east of the Colorado River, we describe it as a new species.