VERTEBRAL COLUMN. 43 



difference between the skeleton of Ichthyopsida on the one hand 

 and Amniota on the other. As a rule the vertebrae of Reptiles 

 become definitely articulated with one another, and are of the 

 procoelous type: the above-named genera, with intervertebral 

 remains of the notochord, form an exception to this rule, as do 

 also Crocodiles and Birds, in which intervertebral disks or menisci 

 exist ; in the latter, however, they are not present in the cervical 

 region. 1 



What has been said as to the classification of the vertebrae into 

 different regions in Amphibia, as well as to the presence of processes, 

 applies here also, though there are always several cervical vertebra} 

 instead of a single one : there are also always at least two sacral 

 vertebras. An atlas, usually consisting of three pieces, and an 

 axis, with an odontoid bone, are always well developed. 2 



The spinous processes of the upper arches vary in size, and 

 transverse processes arise from the centra themselves or close to 

 them. Lower arches (chevron bones) are present in the tail in 

 Lizards, Chelonians, and Crocodiles; and besides these, median or 

 paired inferior processes of the centra themselves are seen in 

 many of the vertebrae of Lizards and Snakes, as well as in Birds, 

 and to some extent in the lumbar region of certain Mammals. 



In consequence of the absence of a pectoral arch, the vertebral column 

 of Snakes and Amphisbsenians, like that of Caecilians, can only be divided into 

 trunk and caudal vertebrae. The vertebral column of Chelonians deserves 

 particular notice as a large portion of it becomes anchylosed with the dermal 

 bones of the carapace, and it is thus rendered immovable in a certain region. 



In Snakes and some Lizards (Iguana) extra articular processes (zygosphenes 

 and zygantra) are developed on the vertebrae. In Lizards small separate ossifi- 

 cations or subvertebral wedge-bones are often present on the ventral 

 side of the vertebral column between the centra ; and in the caudal region, an 

 imossified septum remains in the middle of each centrum, so that the tail 

 easily breaks off at these points. When this happens the tail grows again, but 

 proper vertebrae are not formed. 



In fossil Reptiles, which, both as regards size and number of species, 

 usually surpassed the existing representatives of the group, the sacrum, 

 which gives attachment to the pelvis, often consists of more than two 

 vertebrae, the number being four or five (Ornithoscelida). 



The following facts will give some idea of the monstrous proportions 

 of these old genera of Reptiles : Atlantosaurus immanis, a North 

 American Dinosaur, reached a length of about 80 feet, and its femur was 

 8 feet long and 25 inches thick at its proximal end. The transverse dia- 

 meter of the individual vertebrae amounted to 16 inches, and Apatosaurus 

 laticollis, found in the same strata, possessed cervical vertebrae which 

 reached a diameter of 3^ feet. 



A knowledge of fossil genera of Reptiles is of the greatest 

 interest, as we can see, in many groups, important points of con- 



1 In Crocodiles the vertebrae are mostly procoelous, an exception being seen in the 

 two sacrals and first caudal ; and in Cheloiiiaus there is great variation in the form of 

 the individual centra of the cervical vertebrae, while the thoracic and lumbar have 

 flattened faces, and are firmly united together by cartilage. 



2 The os odontoideum corresponds morphologically to a part of a centrum of the 

 atlas. 



