1) The authors has observed in a boy of 21 years old a supernumerary 3rd premolar tooth the crown of which emerged from the fight lingual side close to the both premolar, mandible. 2) They may have a conical form or be otherwise rudimentary or vestigial, but they do not present ancestal characters. 3) According to the reports of formaers and our experiences about it, the large number of the eruption area was in lingual or palatal side. 4) A third premolar in man could only be accounted for as atavisms by a reversion covering so long a period of time.