XIII 



PHYLUM CHORDATA 



345 



of the cervical, and the shortness of the caudal region. The vertebra? are 

 amphicoelous. The sacrum consists of either one or two vertebrae. There is no 

 sternum. In the skull there are large pre-maxillas ; the bony . palate is absent ; 

 a transverse bone is present. The upper temporal arch alone is developed. 

 There is a well-marked parietal foramen. The ring of bony plates (developed in 



the sclerotic) found in the orbit of some 

 fossil Reptiles is not developed. The 

 pectoral arch (Fig. 954) presents some re- 

 markable features. The coracoids always 



O 



I 



4) 

 O 



O 

 h 

 O 



i 



O 



H 



(A 

 0) 



CO 



FIG. 954. Plesiosaurus, pectoral arch. cor. 

 coracoid ; o. episternum ; til. glenoid cavity ; 

 sc. scapula. (After Zittel.) 



PL>. 



FIG. 955. Plesiosaurus, pelvic arch. 11. 

 ilium ; I*, ischium ; Ph. pubis. (After Huxley.) 



meet in a ventral symphysis, and the 

 ventral portions of the scapula- may also 

 meet. In front is, in most cases, an arch 

 of bone, consisting of a median and two 



lateral portions, which probably represent the episternum and the clavicles : in 

 some forms this is reduced or absent. An obturator foramen is sometimes, but 

 not always, present in the pelvis (Fig. 955). The teeth are implanted in distinct 

 sockets. 



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

 extending onwards to the Cretaceous. 



