This fossil is of the prosoma ("head") of an Eurypterid ("sea scorpion"). This specimen was found at the Ridgemount Quarry in Fort Erie, Ontario, Canada. Its identification might be an Eurypterus lacustris (Harlan). This particular fossil was found in the Fiddlers Green Member of the Bertie Dolestone of the Williamsville Formation. This creature probably existed about 423-418 million years ago (upper Silurian Period). See this website for more details.
Thanks to Dave for sending this fossil to me.