snail ID?

[JoJosReef]

Very flat, smooth pearly bottom with small hole, round dull-looking top of shell. Think it's dead--found it bottom up on sandbed and put it up against glass to see if it would attach, but haven't seen any movement so far. Any ideas?

Now whites:

 

[Spare time]

I'm pretty sure that's the cap of a turbo snail. Could be a different snail cap. The round part is what you see when a snail retreats into its shell

 

[JoJosReef]

I'm pretty sure that's the cap of a turbo snail. Could be a different snail cap. The round part is what you see when a snail retreats into its shell

I have a crowned turbo that died about a week or two ago. Could this be a piece that broke off from it? I also have a smooth turbo which is so far alive and working mostly on the glass but was rock grazing last night.

 

[Spare time]

I have a crowned turbo that died about a week or two ago. Could this be a piece that broke off from it? I also have a smooth turbo which is so far alive and working mostly on the glass but was rock grazing last night.

Its hard for me to say what species died but I was guessing turbo by the size.

 

[WheatToast]

I'm pretty sure that's the cap of a turbo snail. Could be a different snail cap. The round part is what you see when a snail retreats into its shell

+1 This is called the operculum.
From images of Lunella coronata (crowned turbo) online, the opercula look pretty similar to what OP has photographed:

 

[monkeyCmonkeyDo]

+2 it's just the leftovers from a dead snail. They leave behind the shell and the protector piece. Kinda like a shield when u retreat into a hole and hold the shield to cover the hole. Lol.
D

 

[JoJosReef]

+2 it's just the leftovers from a dead snail. They leave behind the shell and the protector piece. Kinda like a shield when u retreat into a hole and hold the shield to cover the hole. Lol.
D

So sad!

My crowned turbo struggled quite a lot--amazingly clumsy, whereas his smooth turbo cousin zips around without a problem. Was always righting him.