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 
Colossochelys 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 vertebra 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 
Chelonioide), the paddle-like feet are 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. Rhynchocephalia.—The only living represeata- 
tive of this order is the Sphenodon 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 vertebra 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 6. Ichthyopterygia.—This order is entirely extinct. 
* See Guenther’s Contribution to the Anatomy of Hatteria. London, 
1867. 
