An accessible, illustrated survey traces the anatomy, classification, and fossil history of reptiles that inhabited freshwater and marine environments, comparing extinct groups such as plesiosaurs, mosasaurs, ichthyosaurs, and thalattosaurians with living turtles, crocodilians, and lizards. It explains skeletal structure and functional adaptations for swimming, reviews geologic distribution and major orders, and discusses evolutionary relationships among reptiles, amphibians, birds, and mammals. Organized by topic and taxonomic group, chapters combine descriptive restorations, fossil evidence, and interpretive summaries to show how terrestrial reptiles invaded aquatic niches and diversified through deep time.