REPTILIA. ICHTHYOSAURIA. 197 



grooves without being ankylosed to the bone. They are 

 confined to the jaw-bones. 



The ribs are long, and the anterior ones have capitula and 

 tubercula. There is no sternum, but the ventral body wall 

 is strengthened by a complex system of abdominal splint ribs. 



The pectoral girdle is strongly developed, the scapulae are 

 narrow, the coracoids broad, and meet ventrally without over- 

 lapping. There are probably no precoracoids, but clavicles 

 and a T-shaped interclavicle are well developed. 



The limbs are very short, and completely modified into 

 swimming paddles. The humerus and femur are both short, 

 while the radius and ulna, tibia and fibula are generally still 

 further reduced to the form of short polygonal bones. 



The digits are formed of longitudinal series of very numer- 

 ous small bones. The number of digits is five, but there 

 sometimes appear to be more owing to the bifurcation of 

 certain of them, or to the addition of marginal bones, either 

 to the radial or ulnar side of the limb. The humerus has no 

 foramen, and both humerus and femur are unique in that 

 they are distally terminated by concave surfaces instead of 

 by convex condyles. The pelvic limb is much smaller than 

 the pectoral. The pelvis has no bony connection with the 

 vertebral column, and all the component bones are small and 

 rod-like. 



The Ichthyosau.ria are confined to beds of Secondary age 

 and by far the best known genus is Ichthyosaurus. 



Order 5. RHYNCHOCEPHALIA. 



This order includes the living Sphenodon (Hatteria) and 

 various extinct forms. The general shape of these animals is 

 lizard-like and the tail is long. 



The vertebrae are amphicoelous or sometimes nearly flat, 

 and the notochord sometimes persists to some extent. Protero- 

 saurus differs from the other members of the order in having 

 opisthocoelous cervical vertebrae. 



