The families of mygalomorph spiders heretofore commonly regarded as occurring in North America north of thirty degrees latitude are these: Ctenizidae, Dipluridae, Atypidae, and Theraphosidae. A study of a number of species indicates that some revision should be made in the definition of some of the families and that two additional names should be added to the above list. These two are the already established names Hexuridae and Acattymidae.
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Chamberlin et al. (1945) studied this question.