Family Brachaucheniidae. Skull long, neck very short, with but thirteen vertebrae, shorter than skull. Cervical ribs singleheaded. Pterygoids not reaching to vomers. Paddles imperfectly known.
Upper Cretaceous. Brachauchenius Williston, North America.
Genera incertae sedis
Triassic. "Plesiosaurus" Conybeare, Europe.
Jurassic. Eretmosaurus Seeley, Colymbosaurus Seeley, Ischyrodon Meyer, Liopleurodon Sauvage, Spondylosaurus Fischer, Simolestes Andrews, Europe. Megalneusaurus Knight, Pantosaurus Marsh, "Muraenosaurus" Seeley, North America.
Lower Cretaceous. "Plesiosaurus" Conybeare, North America.
Upper Cretaceous. Mauisaurus Hector, New Zealand. Polyptychodon Owen, Europe. Cimoliosaurus Leidy, Oligosimus Leidy, Brimosaurus Leidy, Piptomerus Cope, Orophosaurus Cope, Embaphias Cope, Taphrosaurus Cope, Uronautes Cope, "Plesiosaurus" Conybeare, North America.
7. ORDER PLACODONTIA
Temporal opening bounded by parietal, postfrontal, postorbital, and squamosal. Jaws and palatines with few, very large, flat crushing teeth. A parietal opening. Vertebrae amphicoelous, with hyposphene, hypantrum. Ribs double-headed. Remainder of skeleton unknown.
This singular group of littoral, shell-eating reptiles has long been a problem, because of our ignorance of the skeleton. Some would include them among the Sauropterygia as a separate suborder; others would give to them the same rank among the Therapsida. If the supratemporal and interparietal are really present, as believed by Huene, they would certainly find no place among the Sauropterygia. But their presence has been denied. On the other hand, if there should prove to be but a single coracoid on each side in the pectoral girdle, their location among the Therapsida would be improper. Placochelys has a carapace of bony plates, both above and below, with isolated ones upon the skull, all of which seem to be wanting in Placodus. Their presence or absence, however, is of no more importance than in the Dinosauria, or Squamata, as examples.
