THE ICHTHYOSAURS. 511 



the Galapagos Islands, the Mascarine Islands (Mauritius and 

 Rodriguez), and also of the Aldabra Islands, lying northwest 

 of Madagascar, are in some cases colossal in size, the shells 

 being nearly two metres (six feet) in length. The fierce Mas- 

 carine species were contemporaries of the dodo and solitaire, 

 and are now extinct. The bones of extinct similar species 

 have been found in Malta and in one of the West Indian 

 islands. The land tortoises are long-lived and often reach a 

 great age. Certain tortoises of the Tertiary Period, as the 

 Colossochdys of the Himalayas had a shell twelve feet long 

 and six feet high. The turtles extend back in geological 

 time to the Jurassic, a species of Compsemys being char- 

 acteristic of the Upper Jurassic beds of the Rocky Moun- 

 tains. (Marsh.) 



The eggs of turtles, as those of birds, are of large size ; 

 they are buried in June in the sand and left to be hatched 

 by the warmth of the sun. It is probable that turtles do not 

 lay eggs until eleven to thirteen years of age. The develop- 

 ment of turtles is much as in the chick. By the time the 

 heart becomes three -chambered, the vertebrae develop as far 

 as the root of the tail, and the eyes are completely enclosed 

 in their orbits. The shield begins to develop as lateral folds 

 along the sides of the body, the narrow ribs extending to the 

 edge of the shield. In the lower forms of turtles (the 

 ChelonioidcB), the paddle-like feet arc formed by the bones of 

 the toe becoming very long, while the web is hardened by 

 the development of densely packed scales, so that the foot is 

 nearly as rigid as the blade of an oar. 



Order 5. llliynclioceplialia. The only living representa- 

 tive of this order is the Sphenodou or Hatteria of New Zea- 

 land ; a lizard-like form of simpler structure, however, than 

 the lizards in general. This rare creature somewhat re- 

 sembles an iguana in appearance, having a dorsal row of 

 spines. It is nearly a metre (32 inches) in length. In this 

 group the vertebras are biconcave ; the quadrate bone is im- 

 movable, and there are other important characters based on 

 a study of the living and fossil forms, the latter represented 

 by the Triassic Rhynchosaurus and Hyperodapedon. 



Order G. Ichthyopteryyia. This order is entirely extinct. 



