The spinal column and base of the cranium are separate, but the faces have come in contact and partly coalesced. There may be two faces, but they are often partially undeveloped (Fig. 2). The union here, as in the two preceding classes, is anterior, so that it is the faces which come in contact and coalesce. Symmetrical parts of the two faces may thus unite, especially those in the middle line, as the mouths and noses, while the ears are brought close together. There is in some cases a peculiar coalescence of the two faces, as if while facing one another they had become flattened out against each other and the parts carried to either side. There are thus two faces, looking to the right and left, but each face really belongs half to one body and half to the other. This form of Mr. Facing-both-ways is called Janiceps. Sometimes one of the faces is only a rudiment. In all these forms the mouth and tongue are single in their posterior part; the oesophagus, stomach, and duodenum are single. The lungs, urinary and sexual organs, are double, but the heart single. The arms are nearly always completely double.
These three forms are usually included in the genus Thoracopagus, and they constitute a very common form of double monster. Amongst them the second or Sternopagus is the most frequent. In some cases one of the twins is ill developed, and exists as an appendage to the other, forming a parasite attached to the abdomen or thorax. This form is called Parasitic thoracopagus, and is illustrated by the case of Lazarus Colloredo, who was born in 1716, and lived to an adult age. The smaller twin had most of the external parts, with the exception that there was only one leg.
Fig. 2. - Syncephalus. Janiceps. Twins united by head and thorax. The face shown belongs half to one twin and half to the other. There is another face opposite this one, which is in this case imperfectly formed. (Glasgow Hunterian preparation).
It is customary to include under the thoracopagus parasiticus cases in which the head of the ill-developed foetus is absent (acephalic parasites), but this is more probably due to incomplete abcaudal fission. (See further on).