Actinoceras capitolinum was a complex animal called a Nautiloid. Like the modern Nautilus, they had compartentalized shells that could be filled with air to adjust boyancy to control the animals level in the water. The last compartment had a squid-like body with excellent eyes and a sophistocated brain. New compartments were added as the animal grew, and could reach a large size. This species was straight, but some, like the modern Nautilus, were curved.