The Order Chelonia: Modified
Tortoises, terrapins and turtles belong to the Order Chelonia and have an ancestry that is even older than the crocodiles. The strengthened bony plates occurring in crocodiles (ossicles) have in tortoises become modified to form a continuous dorsal carapace and a ventral plastron. This represents the most effective armour developed byhttp://en.wikipedia.org/wiki/Carapace any vertebrate group, and this pattern has changed very little since it first evolved. The turtles reverted to an aquatic life style where the heavy shell was less of an impediment to locomotion. However, the shelled egg, an essential adaptation to terrestrial life, did become an impediment since the membrane beneath the shell by which the embryo breathes through the shell pores functions by gaseous exchange and cannot work in water. Consequently turtles come on to beaches to lay their eggs in a terrestrial environment. However, when the young turtles hatch they have a perilous journey from where they hatched (above spring high tide) to the sea, and many succumb to predation.