204 THE VERTEBRATE SKELETON. 



Suborder (3). PYTHONOMORPHA'. 



This suborder includes Mosasaurus and its allies, a group 

 of enormous extinct marine reptiles found in beds of Cre- 

 taceous age. 



The skin is in most forms at any rate unprovided with 

 dermal scutes. The vertebrae may be with or without zygo- 

 sphenes and zygantra. The skull resembles that of lizards, 

 having an interparietal foramen, and a cranial cavity open 

 in front. The squamosal takes part in the formation of the 

 cranial wall, and the quadrate articulates with the squamosal, 

 not as in Lacertilia with the exoccipital. There are large 

 supratemporal fossae, bounded below by supratemporal arcades. 

 The teeth are large and acrodont, and occur on the pterygoids 

 as well as on the jaws. The two rami of the mandible are 

 united by ligament only. Pectoral and pelvic girdles are 

 present, but clavicles are wanting, and the pelvis is not as a 

 rule united to any sacrum. 



The limbs are pentedactylate, and are adapted for swim- 

 ming, while all the limb bones except the phalanges are rela- 

 tively very short. The number of phalanges is riot increased 

 beyond the normal, and they articulate with one another by 

 flat surfaces. The terminal phalanges are without claws. 



Order 7. DiNOSAURiA 2 . 



The extinct reptiles comprising this order were all terres- 

 trial, and include the largest terrestrial animals known. They 



1 E. D. Cope, Eep. U. 8. Geol. Surv., 1875, vol. n., The vertebrata 

 of the Cretaceous formations of the west. E. D. Cope, P. Boston Soc. 

 1862, xii. p. 250. 0. C. Marsh, Amer. J. Sci. 1872, vol. 3. B. Owen, 

 Quart. J. Geol. Soc. 1877, and 1878. 



2 J. W. Hulke, Presidential address to the Geol. Soc. of London, 

 1883 and 1884. 0. C. Marsh, many papers in the Amer. J. Sci. from 

 1878 onwards, also in the Geol. Mag. E. Owen, History of British 

 fossil reptiles : Dinosauria (Palaeont. Soc.). 



