http://www.wired.com/wiredscience/2009/09/ghostshark/
Perhaps the most intriguing feature of the newly described species, Hydrolagus melanophasma, is a presumed sexual organ that extends from its forehead called a tentaculum.
“They have this club on the top of their head with spikes. People think it’s used for mating,” Long said. “It’s like a little mace with little spikes and hooks and it fits into their forehead. It’s jointed and it comes out. We’re not sure if it is used to stimulate the female or hold the female closer.”
I will let someone else think up a clever comment.