THE RIBS AND STERNUM II9 



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 proc- 

 esses 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. 



Ventral or Abdominal Ribs 



Many, perhaps most, of the Stegocephalia, especially the Branchi- 

 osauria, 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 complex- 

 ity, 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 Aigialo- 

 saurus and some recent lizards; in the Choristodera, Homoeosaurus, 

 Hyperadapedon, and Sphenodon of the Rhynchocephalia; in the 

 Pseudosuchia {Scleromochlus) , Phytosauria, Pterosauria, Crocodilia, 



