Pitted Ceratosoma (Ceratosoma miamiranum)
It has a relatively hard mantle with a series of rounded lobes around the edge. Smaller specimens have many compound tubercles and usually there is a median longitudinal ridge and a series of ridges running out from the longitudinal ridge to the mantle edge. As specimens grow in size these ridges can partially disappear.
Mandati Jetti, Wangi-wangi Island
Photo by me (Lyra)