500 



Comparative Morphology of Chordates 



along the body to the hindleg and tail (Fig. 389). This pair of integu- 

 mentary membranes must have made possible not merely a gliding 

 excursion through air but a positive flight and, judging by the mecha- 

 nism, it was doubtless very effective flying. And so it came about that 

 land, water, and air were dominated by reptiles. They existed in such 

 diversity of forms as is indicated by the fact that their classification 

 requires the making of 30 or more orders. Chief of the land types were 

 the dinosaurs (Figs. 390, 391). Some were no larger than cats and dogs; 

 others ranged up to monsters 100 feet long and weighing 50 tons. Some, 

 with long, strong hindlegs and very short forelegs, must have been 

 bipedal in habit. Some were herbivorous; many were carnivorous. In 

 the water were the externally fishlike ichthyosaurs (Figs. 392, 393), 

 some of them 30 feet long, and even longer plesiosaurs having a long, 

 slender neck and a very small head quite out of proportion to the broad, 

 heavy body (Fig. 392). There is some evidence that ichthyosaurs were 

 viviparous; skeletons of small ones are sometimes found inside the 

 skeleton of a large individual. In various parts of the world were 

 aquatic lizards, mosasaurs, up to 40 feet long. Some of the best speci- 

 mens of them have been found in Kansas, which was at the bottom of a 

 Mesozoic sea. Many of these large aquatic reptiles were carnivorous. 

 The heavy coat of armor worn by the numerous "ganoid" fishes of the 

 time must have been useful. The batlike flying reptiles, pterosaurs 

 or pterodactyls, were abundant in the later Mesozoic, some measuring 

 25 feet across the spread wings. The Rhynchocephalia, Crocodilia, 

 and Chelonia were well established in the early Mesozoic. The Sauria 

 (Lacertilia) appear somewhat later and the Serpentes (Ophidia) 

 not until the latter part (Cretaceous) of the Mesozoic. 



The skulls of ancient reptiles are of four types, as distinguished 

 by differences in the structure of the posterior-lateral (temporal) 

 region. In all cases there is a considerable space on each side between 

 the lateral wall of the brain-case, including the auditory capsule, and 



Fig. 392. Plesiosaurs and ichthyosaurs. ( From a painting by C. R. Knight. Copy- 

 right, Chicago Natural History Museum.) 



