Dermoids which are localized on the limbus and cornea are very unusual congenital tumors from the group of choristomas. Choristomas are congenital masses of tissues not normally present in the location in which they are found. These lesions are present at birth and they occasionally enlarge, especially at puberty. Most likely, tissue destined to become skin is displaced to the surface of the eye during fetal development.Dermoids can occur over the cornea, limbus or conjunctiva. They are round or ovoid, yellowish white, solid vascularized and dome-like. They can involve the central or entire cornea or form a ring around the circumference of the limbus.
Photos and information from here