What is wrong with my snail's shell?

[R.naillon]

I am pretty new to snails and last night when watching the fish I noticed that one of my 3 snails had something wrong with its shell.

 

[Skai]

Is there any source of calcium in the tank for them? Most shell issues that I personally have come across seem to stem from not enough calcium for the snails to grow/maintain nice shells

 

[libertybelle]

New development, or are the snails new? Agreed with the above that a source of ca like cuttlebone will help them recover.