CHAPTER XI 



rlesiosaukia iciitiiyo.sauiua i'tkrosauria i'vthoxomourua 



Sub-Cla ss VII.—PLESIOSA URIA. 



Mesozoic aquatic reptiles, with two pairs of ^^(^ntadacti/le limbs, 

 firmly fixed quadrate bones, single temporal arches, numerous 

 alveolar teeth, and ribs which articulate only vjith the centra 

 of the biconcave vertebrae. 



The Plesiosauria comprise the Mesosauri, Notliosauri, and Plesio- 

 saiiri 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 aliove it 

 shows the nostrils, orliits, very large supratem])oral foramina, 

 and tlie 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 tlie two bones just 

 mentioned with the squamosal mass, which overlaps the greater 

 portion of the quadrate, and perhaps contains tlie quadrato-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 Larlosaurus. The choanae open separately 

 between the vomers and maxillaries. The pterygoids are very 

 long ; posteriorly they join the quadrates, anteriorly they extend 



