Dictionary of Ichthyology
hyperostosis = excessive or abnormal thickening of bone. This is really a relative term as strongly developed and thickened bones occur in fishes naturally and contrast with the usual thinner bones, e.g. in the cranium of grunts (Haemulidae) and croakers (Sciaenidae) which lack suture lines and are very solid, unlike most fish skulls with sutures and discrete bones. Also called Tilly bones, q.v.