podial and epipodial bones, hyperphalangy, and often hyperdactyly. Premaxillae long; maxillae short. A parietal foramen; free paroccipitals, large stapes; no ectopterygoids or dermosupraoccipitals. Teeth inserted in sockets or grooves, labyrinthine in structure; none on palatal bones. The large upper temporal vacuity is bounded by parietal, postfrontal, and tabular (supratemporal). No lateral opening. Vertebrae short, deeply amphicoelous, without persistent dorsal intercentra. Scapulae small; a single coracoid; clavicles and interclavicle present. No sternum, but numerous parasternals. Pelvis more or less plate-like with small pubo-ischiatic vacuity. Prearticular bone of mandible distinct.
The ichthyosaurs were exclusively marine reptiles, more perfectly adapted to aquatic life than any other known ones unless it be the plesiosaurs. They varied from about two to about thirty feet in length.
Family Mixosauridae. Cervical ribs for the most part holocephalous. Tail with a preterminal dilatation, slightly decurved. Chevrons Y-shaped. Epipodials relatively long; feet pentedactylate. Face less elongate. Teeth more or less anisodont, inserted in sockets.
Middle and Upper Triassic. Mixosaurus Baur, Spitzbergen, Switzerland, Germany.
Family Shastosauridae. Body more elongate. Cervical ribs dichocephalous. Tail distinctly expanded and decurved distally. Chevrons Y-shaped. Epipodials relatively long. Feet tetra- or tridactylate.
Both the Mixosauridae and Shastosauridae, which Merriam gives only sub-family values under the Mixosauridae, are more primitive, with less perfect aquatic adaptations than the later forms of the Ichthyosauridae, and especially the Ophthalmosauridae.
Middle or Upper Triassic. Cymbospondylus Leidy, Toretocnemus Merriam, Merriamia Boulenger, Delphinosaurus Merriam, Shastosaurus Merriam, Phalaradon Merriam, California, Nevada. Pessosaurus Wiman, Spitzbergen.
Family Ichthyosauridae. Fewer presacral vertebrae; pelvis more reduced; tail with a broad terminal fin; epipodials shorter; dorsal ribs dichocephalous; chevrons separate or fused; hind limbs
