The North Island takahe (the moho) was first described from subfossil remains in 1848 and named Notornis mantelli. There is a possible record of a live moho from 1894 but otherwise it is unknown apart from skeletal remains.
The South Island takahe was described in 1883 as Notornis hochstetteri from extant specimens (the first of which was caught in 1849). N. hochstetteri was then reduced to a subspecies of N. mantelli, but re-elevated to full specific status (as Porphyrio hochstetteri) in 1996.
The two species are really quite different in proportions, with extinct moho actually being larger and taller than the living takahe but less bulky. They are thought to have derived from two separate colonisations of New Zealand by the swamphen ancestors.
(The Tiritiri Matangi website lists their birds as Porphyrio mantelli mantelli, which even if you aren't splitting the two species is simply wrong)
