Monotremata Bonaparte 1837

Families: 2 families with 3 genera and 5 species:

Family Tachyglossidae Gill 1872 (2 genera with 4 species and 9 subspecies)

Family Ornithorhynchidae Gray 1825 (1 genus with 1 species)

Discussion: Reviewed by Griffiths (1978). The order is the sole extant representative of the Subclass Prototheria (all other living mammals belong to subclass Theria). McKenna and Bell (1997) divided the order into two (Platypoda and Tachyglossa); the date of divergence of the two living families is unknown, and conservatively they are retained here in a single order.