Ornithorhynchidae

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, are four extant mammal species belonging to the Tachyglossidae family of the monotremes, an order of egg laying mammals. Together with the Platypus, they are the only surviving members of that order comprising the only extant mammals that lay eggs...

s. Within Ornithorhynchidae are two genera

Genus

In biology, a genus is a taxonomic unit used in the classification of living and fossil organisms. The term comes from Latin genus "descent, family, type, gender" , cognate with – genos, "race, stock, kin" ..In addition, genus is a taxonomic rank in the hierarchy In biology, a genus (plural:...

, 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
      Ornithorhynchus maximus
      Ornithorhynchus maximus was a platypus from the Late Pliocene to Late Pleistocene. Some scientists believe it is an aquatic version of an echidna whereas others still believe that it is a platypus.-References:...
      
      

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.