Nephtys is feminine therefore Rozbaczylo & Castilla should have modified the suffix to become posterobrancha (& the original Hartman name probably should have been 'posterobranchiatus' as a better adjectival form)
Rozbaczylo, Nicolás; Castilla, J. C. (1974). La familia Nephtyidae en Chile (Annelida Polychaeta). <em>Studies on Neotropical Fauna.</em> 9(2): 179-206. Available for editors [request]