Orthoceras: These very primitive members of the phylum Cephalopoda are similar to the ammonites, with a straight shell. Approximately 400, 000, 000 years old, orthoceras are the ancestral cousin of the modern-day squid or octopus. The word orthoceras means " Straight Horn", and refers to its shape. All orthoceras have chambers separated by walls called septa. They also have a tube that runs down the center of the fossil called a Siphuncle. It is believed that this tube was used to carry gas into the chambers to control the buoyancy of the shell. The Orthoceras from Morocco occurs as white fossils in a dark grey or black marble matrix. The result is a fossil with very good contrast.