Subject: I found a black sea creature on t;he beach. What is it?

I was at Virgina Beach when I found tons of them.  They were black and 
were shaped like a box with four spines, one on each corner.  It had 
something in it too.  It was yellow liquid with lumps and smelled 
horrible.  Some had holes in them where something had come out.  They 
were about an inch thick, three inches long, and two inches wide.  The 
spines were about three inches long but very skinny.    

Re: I found a black sea creature on t;he beach. What is it?