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Description:
66 p. : ill. ; 26 cm. ; Includes bibliographical references (p. 62-66). ; "The neurocranium of the shark Xenacanthus (sensu lato) from the early Permian of Texas shares derived characters with the neurocrania of various other Paleozoic shark genera, including Tamiobatis, 'Cladodus,' 'Ctenacanthus,' Cladoselache, and with Hybodus and the living neoselachians. These characters are consistent with the hypothesis that the above extinct and the living taxa belong to the monophyletic group Elasmobranchii. Xenacanthus and Tamiobatis are proposed as sister taxa related to 'Cladodus' and possibly to 'Ctenacanthus' from the Cleveland shale. The details of Xenacanthus and Tamiobatis cranial morphology are based on three-dimensional specimens, both whole and sectioned'--P. [4].