The inca bone in the human skull is usually regarded as the homologue of the interparietal of some other mammals. The commonest or best-known form of the interparietal occurs in the rabbit as a single somewhat oval bone, its long axis being transverse, filling up a space between the parietals just in front of the line or curve of the occipital.

Carl Vogt (Lectures on Man, London Anthropological Society, 1864) makes frequent and detailed reference to a Helvetian skull. From the woodcuts, p. 52, fig. 15; p. 66, fig. 22; p. 70, fig. 26; p. 389, fig. 124; and p. 390, fig. 125, it is clear that a small round undivided bone occurred in this skull in a situation roughly corresponding with the position of the interparietal in the rabbit. I say “roughly,” because the posterior border of the bone in the Helvetian skull just touches the occipital at the lambdoid suture, while in the rabbit the interparietal for about half the extent of its perimeter is in contact with the occipital.