Ornithorhynchidae is one of the two extant families in the order Monotremata

Monotreme

Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...

, and contains the Platypus

Platypus

The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...

 and its extinct relatives. The other family is the Tachyglossidae, or echidna

Echidna

Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...

s. Within Ornithorhynchidae are two genera

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

, Obdurodon

Obdurodon

Obdurodon is an extinct monotreme genus containing three species. Obdurodon differed from modern platypuses in that it had molar teeth .-Obdurodon dicksoni:...

and Ornithorhynchus:

• Family Ornithorhynchidae
  • Genus †Obdurodon
    Obdurodon
    Obdurodon is an extinct monotreme genus containing three species. Obdurodon differed from modern platypuses in that it had molar teeth .-Obdurodon dicksoni:...
    
    — an ancient branch of the platypus family
    • †Obdurodon dicksoni
    • †Obdurodon insignis
    • †Obdurodon/Monotrematum sudamericanum
  • Genus Ornithorhynchus
    Platypus
    The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
    
    
    • Ornithorhynchus anatinus (the modern Platypus
      Platypus
      The platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
      
      )
    • †Ornithorhynchus maximus

Another two genera, Steropodon

Steropodon

Steropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived during the middle Albian stage, in the Lower Cretaceous period...

and Teinolophos

Teinolophos

Teinolophos trusleri was a prehistoric species of monotreme, or egg-laying mammal. It is known from a lower jawbone found in Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous. It is the earliest known relative of the Platypus.The species name honours the...

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.