Page:The Osteology of the Reptiles.pdf/137

Rh and Pseudosuchia have but that number, and since doubtless the Diapsida began with but two.

Many temnospondylous amphibians have on the hind border of the dorsal ribs an angular uncinate process, like that so characteristic of birds. Such ossifications have never been observed among the older reptiles. They occur in the Jurassic Homoeosaurus and the modern Sphenodon of the Rhynchocephalia; imperfectly ossified processes also occur in the Crocodilia. In many other reptiles they doubtless remained unossified, and in much probability will yet be found in other reptiles as fused processes or separate ossifications.

Many, perhaps most, of the Stegocephalia, especially the Branchiosauria, had on the under side of the body an armature of bony rods, or plates, of various forms, called by Baur gastralia, by Gegenbaur the parasternum, and ordinarily known as ventral or abdominal ribs. They are arranged in a V-shaped pattern with the apex in front, and may sheath the whole under side of the body, extending on the limbs. In some cases, exterior to these a distinct armor of dermal plates has been observed. Among the temnospondylous amphibians they occur more rarely. In Cricotus, an ambolomerous type, they sheath closely the under side of the abdomen, each composed of an unpaired median piece, and numerous lateral ones. Among the Rhachitomi they have been observed in Archegosaurus in the shape of slender rods. They are unknown in the Stereospondyli.

Ossified parasternal ribs, in greater or lesser numbers and complexity, occur in some members of every chief group of reptiles, though far from constantly in each group. Among the Cotylosauria they are known in three families, the Sauravidae, Captorhinidae, and Procolophonidae; they are certainly absent in some, if not many, of the known genera of the order. They have been observed in only a few of the Theromorpha, and are certainly absent in some of the families. They are known in Galechirus of the Dromasauria; among the Proganosauria; Protorosaurus, Kadaliosaurus, Pleurosaurus, and Saphaeosaurus of the subclass here called the Parapsida; in Aigialosaurus and some recent lizards; in the Choristodera, Homoeosaurus, Hyperadapedon, and Sphenodon of the Rhynchocephalia; in the Pseudosuchia (Scleromochlus), Phytosauria, Pterosauria, Crocodilia,