The anatomical differences by which the platypus, and its only allies the echidnas, are separated from all other mammals, so as to form a distinct sub-class, are described in the article Monotremata, where also will be found the main distinctive characters of the two existing representatives of the group. It is there stated that the early stages of the development of the young are not yet fully known.
The echidnas are exceedingly restless in confinement, and constantly endeavour by burrowing to effect their escape.
Of the three-clawed echidnas (Proechidna) confined to New Guinea there are two species, Bruijn's echidna (P. bruijnii), discovered in 1877 in the mountains on the north-east coast at an elevation of 350o ft., and the black-spined echidna (P. nigroaculeata) of larger size - the type specimen measuring 31 in., as against 24 in.
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