94 EVOLUTION IN THE PAST 



evidence that a structural modification to ensure them 

 against this risk was in progress ; but it is not until the next 

 Period that it is found in working order. 



Jurassic crocodiles, although mostly of marine habits, 

 did not, as a rule, venture far out to sea. Some of them, how- 

 ever, were to a certain extent sea-rovers, and had developed 

 fish-like features (Metriorhynchus, Geosaurus). The limbs of 

 these more venturesome animals had become modified into 

 paddles. The fore-limbs, moreover, had been considerably 

 reduced, and exhibited likeness to the pectoral fins of fishes. 

 The hind-Umbs had undergone less transformation, and were 

 of good length, and retained a fair amount of scrambling 

 ability. The animals no doubt enjoyed visiting the shore for 

 a bask in the sun, and periodical visits would be necessary 

 for the females for egg-laying purposes. 



Towards the close of the Period some small crocodiles 

 with broad and comparatively short snouts, and resembling 

 the common crocodiles of to-day, made their appearance 

 (Atoposaurus, Goniopholis). In some of these creatures the 

 disparity between the fore and hind limbs was not so marked 

 as in the case of their long-snouted relations. These short- 

 snouted forms, it is supposed, haunted lakes and rivers, and 

 waylaid little mammals coming down to drink. 



Crocodiles with backbones vertebrated in the modern 

 manner, with a ball-and-socket arrangement, were possibly 

 appearing at this time ; but the evidence is not conclusive 

 (Heterosuchus). 

 CHELONIANS Tortoises had now become very numerous ; and various 

 species of " hidden-necked " (Cryptodira), " side-necked " 

 (Pleurodira), and intermediate forms were to be seen in the 

 lakes and rivers. 



There is no evidence that any tortoises had taken to 

 living permanently on land. On the other hand some of them 

 seem to have been occasionally issuing from the rivers and 

 estuaries, and embarking on sea hfe — setting out in short 

 to found sea-tortoise or turtle communities (Thalassemy- 

 didce). This enterprise, however, was only in a preliminary 

 stage in this Period. 



Long-necked plesiosaurs were now to be seen of much 



