Prehistoric Life Prehistoric Secrets in Stone The Earliest Fossils The Silurian Reef Trilobites Fish The Coal Forests Early Reptiles and Amphibians Sea Reptiles Flying Reptiles The Teeming Seas Hunting Dinosaurs Biggest Dinosaurs Two-Footed Plant Eaters Plated and Armoured Dinosaurs Horned Dinosaurs The Mammal Age Dawns Grassland Mammals The Ice Age	Shell Shapes Some ammonite shells have ribs. Some have a distinct keel. Some, like Harpoceras have both.   Previous: Back Book: Prehistoric Life Section: Prehistoric Chapter: The Teeming Seas