Conjoined twins are identical twins born with their bodies joined at some point and having varying degrees of residual duplication, a result of the incomplete division of the ovum from which the twins developed. Ischiopagus twins have fused lower half of the two bodies, with spines conjoined end-to-end at a 180 degree angle. These twins have four arms; two, three or four legs; and typically one external set of genitalia and anus. Image appeared in "Philosophical transactions", Volume 31-32 Number 377, Firgure 3 & 4, published 1720-23.