Why does my baby have a ridge like bone I can feel across his head?

Skull suture margin. The skull has about 17 growth centers that allow the bony material to grow/expand.Where two bone plates come close together is a suture line. There are prominent ones that run ear to ear and front to back over the top. Sometimes these are prominent in infants with thin hair & are normal. Rarely the ridges signal premature fusion of sutures but your doctor will check for this at checkups.