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                                         STREPTOSAURIA.

   Under this name I have characterized a group of high rank among the Reptiles which is allied to the Sauropterygia. The diagnosis will be as follows.

   The articular processes of the vertebræ, reversed in their directions; viz., the anterior looking downwards, the posterior upwards; the pro-coracoids distinct from the scapula, but confluent with each other and the mesosternum into a simple breast plate. Mandible with symphysis. Pelvic arch present; limbs present. Neural arches of vertebræ coössified with centra.

    The characters of this order are altogether peculiar. They are largely derived from an almost complete specimen of Elasmosaurus platyurus Cope in the Museum of the Academy of Natural Sciences of Philadelphia. The vertebral character may be explained on the supposition that the zygosphen and zygantral articulation is present, and the zygapophysial wanting, or that the obliquity of the faces of contact of the zygopophyses is reversed. The genera known are three,

Cope.]                                              266                            [January 20,

     I. The vertebræ plane, moderately elongate

     Tail very long, compressed; fore limb small; no diapophyses on lumbar region                                                            Elasmosaurus.

     Tail short, depressed; forelimb strong; diapophyses on lumbar vertebræ                                                                     Cimoliasaurus.

     II. The vertebræ with very short anterior-posterior diameter, slightly biconcave                                                       Crymocetus.

     The species indicated are seven, as follows:

                                        Elasmosaurus Cope.

    Elasmosaurus platyurus Cope. Proc. Acad. Nat. Sci., Phil., 1868, 92.

     Length about forty-five feet; bulk of body near that of an elephant. The upper cretaceous of Kansas.

    Elasmosaurus orientalis Cope, MS.

     Dimensions similar to those of the proceeding. The Cretaceous Green-sand of New Jersey.

    Elasmosaurus constrictus, Plesiosaurus constrictus Owen, British Reptiles.

     Known only from a caudal vertebrae from the British Chalk

                                        Cimoliasaurus Leidy

     Cimoliasaurus, Discosaurus, and Brimosaurus Leidy.

    Cimoliasaurus magnus - Leidy, Cretaceous Reptiles N. Am. Discosaurus vetustus Leidy, L.C.

     Cretaceous Green-sand of the Eastern United States.

    Cimoliasaurus grandis Leidy, Brimosaurus grandis Leidy Pro. Acad. Nat. Sci., Phil., 1854, 72.

     Upper Cretaceous of Arkansas.

    Cimoliasaurus latispinus. Plesiosaurus latispinus Owen British Reptiles.

     From the Green-sand of England. Perhaps it is an Elasmosaurus.

                                        Crymocetus Cope.

    Crymocetus Barnardi. Plesiosaurus Barnardi Owen, British Reptiles. Palaeontographical Soc., Cretac. Rept., Tab. XVIII.

     From the chalk of England. This species is founded by Owen on supposed cervical vertebræ. They appeal to me to be rather lumbars, and to indicate an ally of the preceding genera.