It is the sole member of the family Ceratodontidae and one of only 6 lungfish species that exist.    From Wordnik.com.  [WN.com - Articles related to Global climate change causing decline in lizard species] Reference

(1997) Four species of Metaceratodus (Osteichtyes: Dipnoi, family Ceratodontidae) from Australian Mesozoic and Cenozoic deposits.    From Wordnik.com.  [PLoS ONE Alerts: New Articles] Reference