4 CHELONII. 



other. The extent of the dilatation of the ribs greatly in- 

 creases with the growth of the animal ; but in some groups, 

 as Trio7iycidce and the Sea Turtles, they have a part of the 

 distal or outer end not dilated, and always free. 



The lower shield sternum or plastron [scutum ventrale) is 

 formed of four pairs of bones, and an anterior central one, 

 more or less firmly united together, forming a disk, ana- 

 logous to the breast-bone or sternum of other animals. 

 These bones, in the land and most of the fresh -water tor- 

 toises, are united together by toothed sutures into one disk ; 

 but in the genera Ckelydra and in the families Trlonycidce 

 and Chelodlnidce they form a ring, and leave a space filled 

 with cartilage in the centre ; the two lateral pairs being 

 large and often united together, and forming the body of the 

 sternum. These are connected together behind by means 

 an arch formed by the hinder pair : and in front, in the 

 turtle, by the front pair forming an arch with the central 

 bones, as a small pointed process on their inner edge. In 

 the Triovycidcn and Chelydrce, the furcate central bones 

 are united with the linear front pair placed on their outer 

 sides. In the latter family these bones, especially the two 

 lateral pairs, are often covered externally with rugose cal- 

 losities. 



This sternum, in those animals where the bones are closely 

 soldered together, is always attached to the bones of the 

 margin by a dentated bony suture, except in the genus Cistuda, 

 where it is united by a ligamentous junction, admitting of a 

 slight motion ; while in the others, where the bones form 

 a ring (except in the genus Chelydra), they are only united 

 to the margin, when it exists, by some cartilaginous processes. 



The bones of which the sternum is composed are generally 

 so attached to each other as not to allow of any motion ; but 

 in a few of the land and fresh-water tortoises, the junction 

 between the second and third, and sometimes between the 

 third and fourth pairs of bones is by a ligamentous suture, 

 which allows either or both extremities to move on the third 

 pair as a door does on its post, and they are therefore called 

 box tortoises. 



The margin of the shell is usually formed of a series of 

 small bones similar to the cartilages of the ribs in mammalia ; 

 uniting together the ends of the ribs of which the back of the 

 shell is composed, and forming the bond of union between 

 them and the sternum. In some, as among the Trionyches, 

 the marginal bones are reduced to small isolated portions. 



