Cranial and other physical and emotional features
In terms of characteristics of the skull (cranium), the atavistic form included:
A narrow, sloping brow
A strong prominent jaw
High cheekbones
Facial asymmetry
Other physical features included dark skin and existence of extra toes, nipple or fingers.
Besides physical traits, Lombroso suggested there were other aspects of the atavistic form including insensitivity to pain, use of criminal slang, tattoos and unemployment