XIII PHYLUM CHORDATA 335 



golds are so articulated as to permit of free movement. The 

 rami of the mandible are connected together only by elastic fibres 

 at the symphysis, so that they are capable of being widely separated. 

 There is no separate supra-temporal ossification. Sternum and 

 episternum are absent. Movable eyelids and tympanum are 

 absent. 



Includiog all the Snakes— Vipers, Rattlesnakes, Sea-Snakes, 

 Fresh-water Snakes, Tree-Snakes, Blind-Snakes, Pythoas, and 

 Boas. 



Suh-Order c. — Pythonomorpha. 



Extinct Squamata with elongated Snake-like body, provided 

 with limbs which take the form of swimming-paddles. The 

 skull resembles that of the Lacertilia; a supra- temporal helps to 

 suspend the quadrate. The union of the rami of the mandible 

 was ligamentous. There is, as a rule, no sacrum, the ilia not 

 articulating with the spinal column. 



Order II. — Rhynchocephalia. 



Lizard-like, scaly Reptiles with well-developed pentadactyle 

 limbs adapted for walking. The opening of the cloaca is trans- 

 verse. There are no copulatory sacs. The vertebrse are amphi- 

 ccelous, sometimes enclosing vestiges of the notochord. The 

 sacrum consists of two vertebrae. Numerous intercentra are 

 present. The ribs have simple vertebral extremities, and are 

 provided with uncinates (see below). There is a system of 

 abdominal ribs. The quadrate is immovably fixed to the other 

 bones of tlie skull. There are both upper and lower temporal 

 arches. The rami of the mandible are united by ligament. There 

 is a sternum. The teeth are acrodont. The lungs, heart, and 

 brain resemble those of the Squamata. 



This order comprises only a single living genus, Sphenodon or 

 Hatteria, together with a number of fossil forms. 



Order III.— Chelonia. 



Reptilia having the body enclosed in a shell of bony plates, 

 consisting of a dorsal carapace and a ventral plastron, partly of 

 dermal, partly of endoskeletal origin. There is usually on the 

 surface an epidermal exoskeleton of horny plates. The vertebrae 

 and ribs of the thoracic region are firmly fused with the bony 

 carapace, into the composition of which they enter. The quad- 

 rate is immovably united with the skull. The nasal apertures in 

 the skull coalesce into one. The limbs are sometimes terminated 

 by clawed digits adapted for terrestrial locomotion, sometimes 

 modified into the shape of flippers. There are no teeth, and the 

 jaws have a horny investment. The lungs are compound sacs, 



