THE VERTEBRATA 233 



The vertebrae of Ichthyosaurus are common in Jurassic 

 Clays, and easily recognized by their extreme shortness- 

 the length of a vertebral body being only about one 

 quarter its height or breath ; they are biconcave, and the 

 arches are loose. 



Another Mesozoic order of marine reptiles is the 

 Sauropterygia (e.g., Plesiosaimis] distinguished by a long 

 neck and small head (in contrast to the large head and 

 very short neck of ichthyosaurs), by the vertebrae being 

 about as long as high, with very shallow concavities at 

 the ends, and the limbs rather less modified from the 

 land-type. The earliest crocodiles were also marine, but 

 the Chelonia (turtles) had their most striking features 

 developed before they took to the sea. 



Along two distinct lines Mesozoic reptiles adapted 

 themselves to flight. The pterosaurs had a membranous 

 wing, for the support of which the fifth finger was 

 enormously lengthened, the leg and tail also helping 

 to keep it stretched. They lived through the Jurassic 

 and Cretaceous periods. In the birds, on the other 

 hand, there are only a short thumb and two fingers, the 

 latter combining to support the wing but not being 

 greatly lengthened. (In flying Mammals bats yet 

 another type of membranous wing is developed, all four 

 fingers being lengthened to support the wing, like the 

 ribs of an umbrella). The earliest bird (Archteopteryx) 

 was found in the lithographic stone of Solnhofen, high 

 in the Jurassic. The few known Mesozoic birds all 

 have teeth, and Avchaoptevyx alone of all birds has a 

 long, many-jointed tail. 



