JURASSIC PERIOD 95 



larger size some attaining a length of fifteen feet (Crypto- PLESIO- 

 clidus). The limbs had become completely adapted to SAURIANS 

 swimming purposes, and a tail-fin at least in some cases 

 had been developed (P. macrocephalus). Some new forms, 

 characterised by comparatively short necks, were also in the 

 waters (Pliosaurus). These were formidable-looking brutes, 

 attaining in some cases a length of thirty feet, and their 

 teeth long and sharp no doubt occasionally met in the 

 flesh of plesiosaurs. 



Ichthyosaurs or " fish-lizards " were now to be seen forty ICHTHYO- 

 feet in length, and with limbs of more complete paddle-con- SAURIANS 

 struction. Instead of coming to shore for egg-laying purposes, 

 as probably did the earliest ichthyosaurs, the females brought 

 forth their young alive out at sea : and it is doubtful if ichthyo- 

 saurs ever visited the land in Jurassic times. 



Reptile-life had not exhausted its powers in the evolution 

 of swimming, creeping, leaping, and walking forms : for 

 creatures capable of rising from the ground, and navigating 

 the air, had now emerged from the ranks. 



These somewhat bird-like forms are variously known as FLYING 

 " flying lizards," " wing-fingered " animals, and " dragons LIZARDS 

 of the air " (Pterosaurs, Pterodactyls). According to the 

 geological record they made a sudden appearance : but such 

 of course cannot really have been the case. The modification 

 of fore-limbs into organs for holding up folds of skin, and the 

 ultimate adaptation to aerial flight accompanied as it must 

 have been by various bodily modifications represented a long 

 process : and there is no doubt that the known Jurassic 

 forms were preceded by many small and imperfectly winged 

 creatures, of which no trace has yet been found. From what 

 particular group of reptiles they were derived is quite un- 

 known. 



The wing of a flying lizard in being of a membranous 

 substance was like that of a bat ; but its framework was 

 different. It was supported by the " arm " and a greatly 

 elongated " little " finger. The thumb had practically dis- 

 appeared ; but the first three fingers remained, and were free 

 and usable as claws. A bat's wing, on the other hand, is sup- 

 ported by four fingers, and the thumb is the only free digit. 



