Allusions appear frequently in the literature to the idea that a special relationship exists between the African hystricomorph rodents Petromys and Thry-onomys and the Octodontidae and Myocastoridae (Capromyidae auct .), respectively, of South America. I have elsewhere ( In press ) reviewed the history of this arrangement and attempted to show that few positive reasons for classifying these two genera with South American families have ever been advanced. The idea derives ultimately from the unsupported assertion of Waterhouse (1839). Ellerman (1940) classified both Petromys and Thryonomys as sole representatives of the subfamilies Petromurinae and Thryonomyinae of the Echimyidae. This “Echimyidae” is roughly the equivalent of Simpson's (1945) “Octodontoidea,” so these two authors really classified Petromys and Thryonomys in the same way, as Simpson lists them under Petromyidae and Thryonomyidae of the Octodontoidea, stating (p. 212), “… Thryonomys and Petromys are morphologically octodontoid and it has been common practice to refer them to neotropical families and subfamilies, for instance, placing Petromys with Octodon and Thryonomys with Capromys .” Wood (1955) separates Thryonomys and Petromys from the South American hystricomorphs in his splitting apart of the suborder Hystricomorpha. He divides the hystricomorphs into two suborders: Hystricomorpha (Old World forms) and Caviomorpha (New World forms). He still asserts, however, that there is “morphological similarity” between Petromys and Thryonomys and New World families. He says (p. 184), “There are many obvious similarities to the Octodontidae and Echimyidae respectively. These are so great that formerly the African genera were placed in South American families. This is still done sometimes, the latest example being Ellerman, Morrison-Scott and Hayman (1953: 237–239) who consider the Thryonomyinae, Petromurinae and Echi-myinae to be subfamilies of the Octodontidae.” As a matter of fact, Ellerman, Morrison-Scott and Hayman (1953) concede that there may be some doubt about the classification of Thryonomys , …
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