156 



EEPTILIA 



CLASS III 



wanting. The humerus is short and stout, its articular ends expanded, and 

 the distal end with two facettes for the bones of the second segment. Of the 

 latter the radius is distally expanded. The carpals vary in number from 

 seven (arranged in two rows) in Clidastes and Mosasaurus, to one or two 

 rudimentary ossicles in Tylosaurus. There are five metacarpals, differing in 

 shape amongst the various genera, and supporting a variable number of 

 phalanges ; these last being comparatively few in Clidastes and Mosasaurus, 

 and as many as eleven or twelve in the paddles of Tylosaurus. None of the 

 digits are clawed. 



The pelvic arch (Fig. 248) is more weakly developed than the pectoral. 

 The ilium is a slender rod-shaped bone, its superior end free or loosely 

 attached to the single vertebra which functions as a sacrum. The pubes 

 probably do not meet in a ventral symphysis, but the ischia are joined along 



^>^f 



Fig. 24S. 



Platecarpus simus, iarsh sj>. Upper Cretaceous ; Kansas. Pelvic arch and paddles, !/i2- 



(/, Femur; /', Fibula; il, Ilium ; is, Ischium ; rut, Metatarsus; pb, Pubis; t, Tibia; I-V, Digits), (alter Marsh). 



an extensive articular surface. The bones of the hind limb resemble those of 

 the fore, but are more slender, and somewhat smaller. There are probably 

 never more than three tarsals, and sometimes only one. Four or five digits 

 are developed, with a variable number of phalanges. As a rule the posterior 

 paddles are smaller than the anterior pair. 



The limbs of Pythonomorphs and Cetaceans furnish an excellent illustra- 

 tion of the influence of environment in producing analogous structures among 

 widely divergent groups. As a result of their complete adaptation for an 

 aquatic existence, Mosasaurs and whales have developed a very similar form 

 of body. Most of the Pythonomorphs were of large size, and enjoyed a wide 

 distribution, their remains being known from the Upper Cretaceous of Europe, 

 North and South America, and New Zealand. 



The Pythonomorphs were covered externally with scales very similar in 

 size and structure to those of Varanus. These scales have been found 

 impressed upon the bones of the skull, indicating the absence of corneous 

 plates. There were no dermal ossifications of any kind. Small or medium- 

 sized fishes have been observed in the fossilised stomach contents in several 

 instances. 



The earliest discovered cranium, found in the Maestricht Chalk in 1780, 

 was mistaken for that of a Cetacean or Crocodilian until Cuvier demonstrated 

 its remarkable similaritv to the skull of Varanus. Owen united Mosasaurs 

 with lizards under the designation of Lacertilia natantia. The name Pythono- 

 morpha was proposed in 1869 by Cope, who regarded the group as inter- 

 mediate between snakes and lizards. It is argued by Dollo and Williston,. 



