xm 



PHYLUM CHORDATA 



361 



were quite small. They had a lizard-like body, a very long neck, a 

 relatively small head, and a short tail which supported a vertical caudal fin ; 

 the limbs were modified to form swimming-paddles. In older and less special- 

 ised members of the group, however, the limbs were not paddle-like, but 

 adapted for walking. 



The spinal column of the Sauropterygia is characterised by the great 



length of the cervical and the short- 

 ness of the caudal region. The verte- 

 brae are usually amphicoelous. The 

 sacrum consists of either one or two 

 vertebrse. There is no sternum. In 

 the skull there are large premaxillse ; a 

 bony palate is absent ; a transverse 

 bone is present. One temporal arch 

 only is developed. There is a well- 

 marked parietal foramen. The ring of 

 bony plates (developed in the sclerotic) 



sc - 



FIG. 1024. Plesiosaurus, pectoral arch. 

 cor. coracoid ; e. episternum ; sc. scapula. 

 (After Zittel.) 



found in the orbit of some fossil Rep- 

 tiles is not developed. The quadrate 

 is not movable. The pectoral arch 

 (Fig. 1024) presents some remarkable 

 features. The coracoids always meet 

 in a ventral symphysis, and the 

 ventral portions (acromial processes) 

 of the scapulae may also meet. In 

 front there is, in most cases, an arch of 

 bone, consisting of a median and two 

 lateral portions, which probably repre- 

 sent the episternum and the clavicles : 

 in some forms this arch is reduced or 

 absent. In the pelvis the broad pubes and ischia meet in the middle line : 

 the two symphyses may remain separate (Fig. 1025). or they may unite so 

 as to divide the space into two separate obturator foramina. The teeth 

 are implanted in distinct sockets. 



The Sauropterygia date back to the Trias, and perhaps to the Permian, 

 extending onwards to the Cretaceous. 



