Of the vaejovoid families, only the Vaejovidae is entirely North American in distribution. Three of the 10 genera in this family are monotypic and known only from Baja California.
The family Iuridae has two genera that are endemic to North America, but others are known from South America (two genera) and the northeastern Mediterranean area (two genera).
The family Superstitionidae is known mostly from North America, but the monotypic genera from Colombia and from Spain and France also belong to this group. Curiously, all the genera of Superstitionidae except Superstitionia are troglobitic.
The family Chactidae is represented in North America by one monotypic genus endemic to Baja California Sur. The remaining genera of this family are distributed in South America.
The family Euscorpiidae contains four genera, three of which occur in North America. The nominate genus is widespread in the Mediterranean region. A third chactoid family, Scorpiopsidae, is distributed from the Himalayas to Indonesia in southeastern Asia.
The family Diplocentridae contains eight genera and has a disjunct distribution in the New World and southwestern Asia (two genera). The genus Diplocentrus Peters, 1862 is very diverse in Mexico but also has representatives in Belize, Guatemala, and Honduras. The genus Bioculus is restricted to Baja California Sur, Mexico. The genus occurs in Central America and the Antilles. The remaining diplocentrids (four genera) are found in South America and the Antilles.
Only one of the more than 50 genera contained in the Buthidae is found naturally in North America. Centruroides Marx, 1890 is also found through out Central America and the Antilles, and parts of northern South America. It is the only medically important genus in North America. The buthid genus Darchenia Vachon, 1977, described from Merida, Yucatan, Mexico, most closely resembles Old World genera such as Vachon, 1949 and Birula, 1908, and may not be a natural part of the Mexican fauna. I have included it in the key on the chance that it may be collected again. The genus Tityus Koch, 1836 (Buthidae) is not found in North America but is included in the key because it is occasionally introduced on produce and other goods imported from Central and South America.
The key has little utility outside the context of North America. To facilitate species level identifications, useful revisionary works have been annotated in the checklist. These citations, however, are by no means exhaustive. The numbers of species given in the checklist refer only to the North