The rocks have yielded 165 species of extinct elephant; only two species roam the planet today.
The tulip tree (Liriodendron) is in the same family but contains only two species.
There are only two species found in the UK: the Arctic or northern cowrie and the European or spotted cowrie.
There are currently only two species known in this group and they are therefore close relatives of one another.
He is convinced that the people running most networks see only two species of people: "moderates and right-wing nuts".
If there are only two species of elephant, African and Indian, losing one matters a lot.
Among birds, only two species are known in which adult males may have permanent female plumage.
Ten years later, only two species, the bald eagle and the river otter, had fully recovered, while 10 had shown no significant recovery.
The only two species that aren't doing so well this year are oak and ash," said Kate Lewthwaite, a phenology expert at the Woodland Trust.
The genus Didymeles has only two species (D. integrifolia and D. perrieri), both of which are endemic to Madagascar.
Presently, only two species, B. griffithiana Warb.
