slender, with short heads. The vertebræ were firm, closely articulated with the best system of interlocking of any of the mosasaurs. The limbs were flexible and strong, with closely articulating bones and fully developed tarsus and carpus. The aggregate of these characters indicates the most snakelike form and method of progression through water of all the mosasaurs. The genus Clidastes was founded by Cope in 1869, but may ultimately give way to the genus Mosasaurus of Conybeare. Cope's views of Clidastes conclude that the animals were not as large as those of the genus Liodon (Owen), but more elegant and flexible, with an additional
Restoration of the Plesiosaur by Prof. S. W. Williston.
pair of articulations at either end of each vertebra—the zygosphenes—to prevent dislocation by contortions. A larger and still more elegant species was Clidastes tortor (Cope), with lithe movements which enabled it to capture fish by means of its knife-shaped teeth, which were very numerous. Tortor was very slender, with a long and lance-shaped head. It was upward of twenty feet in length, with a head two feet and a half long, the vertebral column elongate and the head narrow and pointed.
The second-type mosasaur perfected by Williston is Platecarpus coryphæus (Cope). Its special characteristics are a short muzzle, slender vertebræ, and an imperfect interlocking zygosphene. The hind paddles are smaller than those forward, but thought to have been more powerful propelling functions than those possessed by other genera. A type skeleton measures fourteen feet, and may have been a young animal. The teeth were very curved and pointed, and formed effective weapons. The neural spines, not closely connected, indicate flexibility. The general characters suggest a power-
