Wormian bones are irregular isolated bones in the skull. Here, we see a human skull with Wormian bones (in yellow) compared to a normal skull on the left. They have a tendency to be more or less symmetrical on the two sides of the skull, and vary in size. Their number is generally limited to two or three, but more than a hundred have been found in the skull of an adult hydrocephalic subject.Image credit: Mariana Ruiz