order ii SQUAMATA 157 



however, that Pythonomorphs could not have been ancestral to the modern 

 Squarnata, since it is impossible to suppose that forms which became so highly 

 specialised and completely adapted to an aquatic existence, should have ever 

 again reverted to a generalised condition, and reassumed terrestrial habits. 

 Pythonomorphs arose undoubtedly from land animals, and may have sprung 

 from Dolichosaurian ancestors ; but they appear to have passed completely 

 out of existence near the end of the Cretaceous, without leaving any progeny. 

 Their extreme differentiation is attested by the loss of a sacrum, multiplica- 

 tion of phalanges, degeneration of the hind paddles, development of a man- 

 dibular joint, and other characters, of which no heritage remains amongst 

 modern Squarnata. 



Family 1. Mosasauridae. 



This family, with the characters of the sub-order, comprises three rather 

 divergent groups, which are considered of sub-family value. Restricted to the 

 Upper Cretaceous. 



Sub-Family A. Tylosaurinae. "Williston. 



Hind limb functionally pentadactyle. Trunk short, tail 'proportionately long. 

 Premaxillae produced into a long rostrum in advance of the teeth ; quadrate with 

 short suprastapedial process. Carpus and tarsus almost wholly unossified; phalanges 

 numerous. Vertebrae without, or with very rudimentary zygosphenes. 



Tylosaurus, Marsh (Liodon, Cope ; Macrosaurus, Owen ; Rhinosaurus, Marsh). 

 Skull - 7 to over 1 m. long. Superior surface of parietal smooth and produced 

 to the posterior margin in the median line, with nearly parallel sides ; foramen 

 at its anterior margin. Postfrontal and prefrontal meeting on the superior 

 border of the orbit ; quadrate stout, with very short suprastapedial process ; 

 maxilla with about thirteen large teeth. One or two carpal and tarsal bones 

 present, not articulating with adjacent elements. Coracoid not emarginate ; 

 limbs about equally developed ; phalanges numerous ; fifth digit elongate. 

 Haemapophyses loosely articulated ; no zygosphenes. Niobrara Group ; 

 Kansas, New Mexico, and Texas. T. proriger, T. dyspelor, Cope. 



Hainosaurus, Dollo. Skull upwards of 1*5 m. long. Frontal broad; pre- 

 frontal and postfrontal meeting over the orbit ; teeth of unequal size. Carpals 

 reduced in number ; phalanges fewer than in Tylosaurus, and paddles relatively 

 larger. Haemapophyses loosely articulated ; cervicals with small zygosphenes. 

 Upper Cretaceous ; Belgium. 



Sub-Family B. Platecarpijstae. Williston. 



Hind limb functionally pentadactyle. Trunk short, fail proportionately long. 

 Premaxillae short, very obtuse, and projecting but little in front of the teeth. Quadrate 

 large, with a long suprastapedial process. Carpus and tarsus imperfectly ossified. 

 Vertebrae' with rudimentary (or functional?) zygosphenes. Interclavicle probably 

 present in all forms. 



Platecarpus, Cope (Lestosaurus, Marsh; ? Holcodus, Gibbes), (Figs. 248-250). 

 Total length about 5 m., of which the skull forms one-eighth. Superior sur- 

 face of parietal small, triangular, not reaching beyond the middle of the bone ; 

 large oval foramen at its anterior border. External nares much dilated 



