LIFE IN SECONDARY TIMES 275 



in the Lower Cretaceous. When they became aquatic their 

 body elongated, like that of the Snake, the limbs, however, 

 retaining the essential characters of land Reptiles, except that 

 they had shrunk. Their bones became shorter and natter, and 

 the whole limb thus became a swimming blade. The oldest of 

 these was Doliclwsaurus of the English Lower Cretaceous, 

 which was only one metre in length. The rami of their 

 mandibles were united. They had supplementary articulating 

 apophyses on their vertebrae, like Snakes. Acteosaurus of 

 Istria was similarly endowed, whereas in Plioplatecarpus of the 

 Upper Cretaceous of Holland these apophyses were absent. 

 In the Mosasaurians the resemblance to Snakes was accentuated 

 by the substitution for the mandibular symphysis of a 

 ligament permitting the separation of the two rami of the 

 lower jaw. The Mosasaurians, whose name means the Lizard of 

 the Meuse, were able to attain a length of six or seven metres, 

 the average length of a Boa or Python. In the Museum at 

 Brussels, there are beautiful complete specimens of Clidastes, 

 which was still longer and more slender. They have been found 

 in Europe and in North America, that is to say on the coasts of 

 the former North Atlantic Continent. Platecarpus and Liodon, 

 however, have an even larger area of distribution. Their 

 bones have been collected from North America and 

 Europe to New Zealand. Liodon haumuriensis of this region 

 attained a length of thirty-five metres. On account of the 

 distensibility of its lower jaw and the specially articulated 

 apophyses of their vertebrae, the Mosasaurians have been 

 regarded as the ancestors of Snakes. Yet though the presence 

 of two posterior rudimentary feet in pythons may prove that 

 the Ophidians are descended from animals with feet and that 

 these feet gradually disappeared in the Scincoidians, from the 

 Skinks to the Slow-worms through the intermediate form 

 Seps, we are still unable to tell how the order of Ophidians 

 arose. 



The Chelonia date back to the Triassic, where they are repre- 

 sented by the genera Chelyzoon, Arctosaurus, Psammochelys, 

 and Progomochelys, which seem to bear some relation to the 

 Rhynchocephala and Crocodilians. Possibly they had a 

 common ancestor with the Theropoda in Primary Times. 

 Hans Gadow has attempted to reconstruct the first Chelonian 

 by attributing to it the more general and apparently primitive 



