Looks very much like a species of Natica to me.  My collection has several and the umbilicus can vary from staircase like (Natica lineata) to lumps and slits as in Neverita didyma which is very common. The nearest specimen I have is Natica gualteriana. My specimen was identified by Kathy Smythe who was the Southern Arabian expert at the time and I believe worked in the British Museum. 

I have some photos but not sure how to up load them. While the shape is not exactly the same and my specimen looks a little less heavy, I think it does fall in the Naticidae.

I collected mine on Seeb beach in 1986. Now sadly nothing but a concrete jungle. I've looked in The Encyclopedia of Shells (Peter Dance) but he doesn't have many examples of Natica. I've only sent small files but can increase if you can't see them properly.