A pterygium is a thickened layer of tissue that grows onto the clear, outer layer of the eye called the cornea. The thickened tissue forms from the conjunctival layer. The conjunctiva is the normal, transparent membrane that covers the outer surface of the eyeball, excluding the cornea. This growth is more often found on the nasal (inner) side of the cornea. This is the result of sunlight crossing into the eye from the more exposed lateral (outer) side and absorbing onto the nasal side.

http://www.newvisioneyecenter.com/cfiles/procedures_pterygium.cfm

http://www.eyemackay.com.au/pterygium-removal.html