CHAPTER XI 



PLESIOSAURIA ICHTHYOSAURIA PTEROSAURIA PYTHONOMORPIIA 



SUB-CLASS VIL PLESIOSAURIA. 



Mesozoic aquatic reptiles, with two pairs of pentadactyle limbs, 

 firmly fixed quadrate bones, single temporal arches, numerous 

 alveolar teeth, and ribs which articulate only with the, centra, 

 of the biconcave vertebrae. 



THE Plesiosauria comprise the Mesosauri, Nothosauri, and Plesio- 

 sauri in an ascending order of development, which concerns 

 especially the changes from a semi -terrestrial to an absolutely 

 aquatic life ; elongation of the neck with corresponding 

 shortening of the tail, and the gradual transformation of the 

 limbs into hyperphalangeal paddles. 



The skull varies considerably in length. Seen from above it 

 shows the nostrils, orbits, very large supratemporal foramina, 

 and the interparietal hole. The nostrils lie rather far back, in 

 front of the orbits, between the elongated premaxillaries, short 

 nasals, and the usually large maxillaries. The orbits are rather 

 small, bordered behind by the postfrontals and postorbitals, 

 which two bones fuse together in the Plesiosauri. The temporal 

 bridge is long, and is formed by the junction of the two bones just 

 mentioned with the squamosal mass, which overlaps the greater 

 portion of the quadrate, and perhaps contains the quadra to- jugal. 

 The dorsal branch of the squamosal joins a corresponding diverg- 

 ing branch of the parietal, and completely shuts off the posterior 

 region of the supratemporal foramen. The interparietal hole is 

 small and placed far back. The palate possesses a row of teeth 

 on the pterygoids in Lariosaurus. The choanae open separately 

 between the vomers and maxillaries. The pterygoids are very 

 long ; posteriorly they join the quadrates, anteriorly they extend 



