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Rh the Athecae), the Pleurodira, Amphichelydia, and Trionychoidea. The definitions of the Athecae and Thecophora given by Hay are as follows:

Athecae. Turtles which retain the primitive dermal armor, with at least traces of the subdermal expansions connected with the ribs. A single living species, Dermochelys coriacea.

Thecophora. Turtles in which the primitive dermal armor is obsolete or abolished, the carapace formed by expansions of the ribs, neural plates, and usually peripheral plates.

Evidences of this primitive external series are found as vestiges, perhaps, of various Cretaceous Thecophora, but they have been accounted for in other ways. If however, the interpretation of the characters of Eunotosaurus, as given on a preceding page, is correct, this theory is much strengthened, since the Eunotosauria have both the expanded dermal ribs corresponding with the ordinary carapace of the turtles, and an overlying carapace composed of rows of plates like those of Dermochelys.

Mesoplastra present. Nasals and lacrimals distinct. Skull wholly roofed over. Neck short, not retractile, the cervical vertebrae amphicoelous or concavo-convex.

. Vomer and parasphenoid with small teeth. Nine dorsal ribs (costals) and seven cervical vertebrae, with ribs. Neck with free dorsal plates. Scapular girdle with short proscapular process or acromion; coracoid short, more or less fused with scapula; a supracoracoid foramen. One sacral vertebra. (Jaekel.)

Upper Trias. Stegochelys Jaekel, Proterochersis Fraas, Proganochelys Baur, ? Chelyzoum Meyer, Germany.

. Cervical vertebrae amphicoelous. Skull elongated; coracoid distally expanded. Carapace united to plastron by narrow buttresses.

Upper Jura. Platychelys Wagler, Pleurosternum Owen, Europe.

Cretaceous. Glyptops Marsh, North America. Helochelys Meyer, Germany.