If it really was Casey's own skull, not just a fake representative of him to put the mask on...
Now, where is the side story that covers the whole question about Raph
keeping the head of his dead friend? 
Did he go back and retrieve it later after nature had time to take care of the rest? Or did he just keep his friend's decomposing body and/or separated head upon his death? ("Weekend at Casey's"?)
Wtf Raph, who does that.
Happy Halloween.