A2\f ATOMY OF THE TUBTLE. 



505 



ing forms, yet they have some afBnities with the Batrachia. 

 They are distinguished from the other reptiles by the shell, 

 the upper part forming tlie carapace, and the lower the 

 plastron ; these two parts unite to form a case or box within 



Pig. 444.— Skeleton of Buropean Tortoise, with tlie plastron or under shell removed 

 -After Owen. 



which the turtle can retract its head and limbs and tail. 



Owing to the presence of the carapace, the dorsal vertebrae 



are immovable, and the ribs do not move upon the vertebrae. 



The bones of the ventral shield or plastron are usually 



