Carapace and Plastron:
The trunk of chelonia is enclosed by Shell which has two parts, a dorsal carapace and a flat ventral plastron, joined on the sides. This is called tortoise shell.
It includes a median serieiof 8 small neural plates. On their right and left lateral sides 8 costal plates are present. On the margins of small 1.1 marginal plates are present. The neural plates are fused with the neural spines of 2nd to 9th trunk vertebrae. The costal plates are fused with ribs of the 2nd to 9th trunk vertebrae. At the anterior end a nuchal plate present and at the hind end a pygal plate is present.
It includes median entoplastron which êorresponds to the interclavicle and a four paired plates known as 1) epiplastra which lie in front of the entoplastron 2) hypoplastron 3) hyoplastra and 4) xiphiplasfronananged below the endoplastron serially.
This shell covers and protects the body of a chelonian. When the animal is disturbed the entire animal withdraws into the shell.