THE SKELETON OF REPTILES 29 



or dilated above for the support of an exoskeleton, or it may be 

 heavy and massive for the attachment of strong muscles and liga- 

 ments. In the modern basilisk lizards and in the ancient Dime- 

 trodon and Edaphosaurus from the Permian rocks of Texas these 

 spines are of enormous length, some of them nearly four feet long in 

 reptiles not twice that length. Slender crawling reptiles usually 

 have no spines, or only vestigial ones. On the sides of the arch 

 there may be a distinct transverse process for the articulation of 

 the rib. 



In all early reptiles the ends of the body or centrum are concave, 

 as they are in nearly all fishes. Such a conformation, called amphi- 

 coelous, gives great flexibility to the spinal column, but only 

 moderate strength, since the intervening spaces are filled with 

 cartilage in life. In all living reptiles, with few exceptions, the 

 body is concave, hke a saucer, in front and correspondingly convex 

 behind, and the intervening cartilage has largely disappeared. 

 Such a mode of union, called procoelous, adds greatly to the strength 

 of the backbone, enabling it to receive greater shocks or greater 

 pressure without dislocation; or to sustain the greater strain of 

 muscles used in running swiftly or in climbing. Among living 

 reptiles, only the gecko lizards and the tuatera have biconcave 

 vertebrae. Some extinct reptiles, such as some of the dinosaurs, 

 animals that walked erect upon their legs, had their vertebrae 

 convex in front and concave behind (opisthocoelous). Birds, 

 though walking erect, have a very different and more complicated 

 articulation of the cervical vertebrae, and certain reptiles, hke the 

 turtles, have very complicated cervical vertebrae. 



In the embryos of all vertebrate animals there appears first 

 an elongated fibrous rod, called the notochord, in the place of the 

 future spinal column. This rod may persist through life, never 

 ossifying, as was the case with all the earUest fishes, and is the 

 condition in some living ones. As the embryo grows, however, 

 the separate segments, or vertebrae, ossify about this rod in all 

 reptiles, forming bony rings, perforate at first in the middle for 

 the more or less constricted notochord. This stage was the 

 permanent condition in all the earliest reptiles and in some later 

 ones. Such animals are said to have notochordal vertebrae, the 



