The Mastixiaceae (Cornales) were more widespread and diverse in the Cenozoic than they are today. The fossil record includes fruits of both extant genera, Mastixia and Diplopanax, as well as several extinct genera. Two of the fossil genera, Eomastixia and Mastixicarpum, are prominent in the palaeobotanical literature, but concepts of their delimitation have varied with different authors. These genera, both based on species described 93 years ago by Marjorie Chandler from the late Eocene (Priabonian) Totland Bay Member of the Headon Hill Formation at Hordle, England, are nomenclaturally fundamental, because they were the first of a series of fossil mastixioid genera published from the European Cenozoic. In order to better understand the type species of Eomastixia and Mastixicarpum, we studied type specimens and topotypic material using x-ray tomography and scanning electron microscopy to supplement traditional methods of analysis, to improve our understanding of the morphology and anatomy of these fossils. Following comparisons with other fossil and modern taxa, we retain Mastixicarpum crassum Chandler rather than transferring it to the similar extant genus Diplopanax, and we retain Eomastixia bilocularis Chandler [=Eomastixia rugosa (Zenker) Chandler] and corroborate earlier conclusions that this species represents an extinct genus that is more closely related to Mastixia than to Diplopanax.