The present study describes the post-hatching development of the external features of the platypus. Specimens range in age from the day of hatching through to 6 months old, and provide the first comprehensive view of the developmental sequence of these features. Various features, those specific to the platypus, those specific to monotremes and those shared with marsupials and eutherians, are described. Features specific to the platypus, including the bill and webbing of the forefeet, are seen to develop precociously. Many features show similarities to marsupials, although marsupials show differential development both in timing and in morphology. The developmental progression is related to the age, in days, although the exact age of the specimens is unclear, and relies on ages given to the specimens at the time of collection, sometimes as long as 70 years ago. Despite this, the progression of development of these features suggests that the ageing given is relatively accurate.