870 HISTORICAL GEOLOGY. 



peau species of Cretaceous Labyrinthodonts are yet reported. The species 

 were too few and too largely terrestrial to have secured frequent fossiliza- 

 tion. 



Reptiles. — The Reptiles of the Cretaceous are for the most part a con- 

 tinuation of Jurassic types, without marked evidence of upward progress. 

 The Horned Dinosaurs, or Ceratopsids of Marsh, probably the latest of the 

 larger species, while showing striking advances toward Mammalian forms 

 in the bovine or rhinoceros-like horns and the two-pronged teeth, are a 

 degenerate group, specialized downward, not upward. As Marsh states, 

 they have the largest heads and smallest brains of any of the Eeptile 

 race. 



The Mosasaurids also, exclusively Cretaceous species, illustrate profound 

 degeneration. For, in these Snake-like species, the Lacertian type becomes 

 enormously multiplicate posteriorly in the vertebral column; the legs are 

 reduced to fins, as in Plesiosauriaus, the posterior part of the body is turned 

 into a fish-like skulling organ, and made the chief means of locomotion ; and 

 the pelvic girdle has lost connection with the vertebrae, there being no sacrum. 

 Here degeneration has developed, not imperfect limbs and a defective skele- 

 ton, not something between a Fish or Amphibian and a Reptile, but a pro- 

 foundly decephalized Reptile, adapted to aquatic life as if its outcome. The 

 last of the Mosasaurs in America occur in the Montana Cretaceous ; in Europe, 

 in the beds of Maestricht. 



Snakes are known from the American Laramie, and also from the 

 Cretaceous of France. They were no doubt successors to an aquatic type, 

 and related, it is supposed, either to the Mosasaurs, or to the Dolichosaurs of 

 the English Chalk. 



The true Crocodilians have a heart of four cavities, and traces of a 

 diaphragm ; and the teeth are implanted in sockets. But these high charac- 

 teristics lose part of their apparent significance in view of the fact (1) that 

 the four-cavity heart, after all, does not prevent the commingling of the 

 venous and arterial blood before it enters the system ; (2) that the character 

 of teeth in sockets began in the Permian ; and (3) that the animal has limbs 

 so short that it "drags its body somewhat along the ground," in true 

 Reptilian style. 



The Dinosaurs, on the contrary, stood on long limbs like a Mammal, and 

 had nearly the same freedom of locomotion. They were, however, as has 

 been explained, merosthenic Reptiles, that is Reptiles having great and 

 powerful hind limbs as the chief organs of locomotion, with usually small 

 fore limbs and small brains. If they were the highest of Reptiles, then the 

 Reptilian type reached its perfection under a merosthenic structure. 



But the distinction of highest, as remarked on page 797, probably belongs 

 to the Pterosaurs, which are eminently prosthenic. The largest species of 

 the group existed in the Cretaceous period. It is not improbable that 

 they had a double heart, like the Crocodiles, and one as good as that of the 

 Birds. 



