In this the union is limited to the region of the sacrum and coccyx. There are two individuals who may live many years, as in the case of the Hungarian girls, Judith and Helena, who lived to the age of 22 years. There may be a common sacrum and coccyx, but even these may be more or less divided and the rest of the skeleton is separate in each. The essential distinction from the Ischiopagus is that each individual has a separate umbilicus. The pelvic organs are less united, but there is a common anus.

The Acardiaci or Omphalosites might be logically included under the group of twin malformations. They consist in ill-formed monstrosities which are born along with well-developed fœtuses. The two are separate but the defective one is connected with the umbilical cord or placenta of the other. It may be more suitable to place these forms amongst the malformations by defect.