Ornithorhynchidae is one of two families in the order Monotremata, and contains the Platypus and its extinct relatives. (The other family is the Tachyglossidae, or echidnas.) Within Ornithorhynchidae are two genera, Obdurodon and Ornithorhynchus:

• Family Ornithorhynchidae
  • Genus Obdurodon -- an ancient branch of the platypus family
    • Species Obdurodon dicksoni (extinct)
    • Species Obdurodon insignis (extinct)
    • Species Obdurodon/Monotrematum sudamericanum (extinct)
  • Genus Ornithorhynchus
    • Species Ornithorhynchus anatinus (alive but threatened: the modern Platypus)
    • Species Ornithorhynchus maximus (now extinct))

Another two genera, Steropodon and Teinolophos may also belong to Ornithorhynchidae. However, they are both very ancient, and scientific literature seems to indicate that they are closely related to, but not a part of, the Ornithorhynchids.

References

Ornithorhynchidae | Ornitorinc