CLASS 3 .-- Union of two distinct fetuses by an osseous junction of the cranial bones.

CLASS 4 .-- Union of two distinct fetuses in which one or more parts are eliminated by the junction.

CLASS 5 .-- Fusion of two fetuses by a bony union of the ischii.

CLASS 6 .-- Fusion of two fetuses below the umbilicus into a common lower extremity.

CLASS 7 .-- Bicephalic monsters.