256 THE OSTEOLOGY OF THE REPTILES 



podial and epipodial bones, hyperphalangy, and often hyperdactyly. 

 Premaxillae long; maxillae short. A parietal foramen; free paroccip- 

 itals, 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, postf rental, and tabular (supratemporal) . No lateral open- 

 ing. Vertebrae short, deeply amphicoelous, without persistent dorsal 

 intercentra. Scapulae small; a single coracoid; clavicles and inter- 

 clavicle present. No sternum, but numerous parasternals. Pelvis 

 more or less plate-like with small pubo-ischiatic vacuity. Prearticu- 

 lar 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 holo- 

 cephalous. 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 Shastosaueidae. Body more elongate. Cervical ribs 

 dichocephalous. Tail distinctly expanded and decurved distally. 

 Chevrons Y-shaped. Epipodials relatively long. Feet tetra- or tri- 

 dactylate. 



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. Cymhospondylus Leidy, Toretocnemus 

 Merriam, Merriamia Boulenger, Delphinosaurus Merriam, Shaslo- 

 saurus Merriam, Phalaradon Merriam, California, Nevada. Pesso- 

 saurus 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 



