of the Bones of the Sternum of Chelonians. 171 



the front part produced forwards, the hinder to the side^ and 

 attached on the inner side to the elongate arched odd bone. 



The Turtles (Chelonia) have the front and hinder pairs of 

 bones narrow, and the front pair furnislied with an elongate, 

 more or less lanceolate, odd bone at the posterior end of the 

 suture between the front pair. 

 * The two lateral pairs of sternal bones being expanded and 



more or less united in the Turtles. 

 ** The two lateral pairs of sternal bones linear and far apart 



in the Luth. 



The Mud-Tortoises and the Luth are peculiar among tor- 

 toises for being covered with a soft leathery skin instead of the 

 horny plates peculiar to this group of animals : but the Mud- 

 Tortoises have beneath their skin more or less dilated callosities, 

 forming their ribs and sternum into a solid mass ; while in the 

 Luth the ribs and sternal bones are very slightly developed, 

 separate from each other, being chiefly supported by the hard 

 callosities enclosed in the skin, so that it may be regarded as 

 a reptile on the border of the vertebrate kingdom. 



On the Osteology of Sphargis &c. 



In the adult Sphargis the bones are not more developed, 

 considering the size of the animal, than they are in the very 

 young (previously described), and very unlike the skeleton of 

 other Chelonians. There is no regular dorsal or sternal shield, 

 nor marginal bones. The vertebrae are compressed ; the seven 

 ribs on each side are depressed, weak, of nearly the same width 

 the whole length, and quite separate from each other, and 

 without any bony expansion between them to form a dorsal 

 disk as in other Chelonians. In all the other very young tor- 

 toises I have seen, the ribs are lanceolate, more or less dilated 

 near the vertebral column ; and it is from the upper surface of 

 this dilatation that the callosities of the outer surface by which 

 the ribs are united commence and gradually proceed down the 

 ribs to the marginal bones. 



The sternum of the adult specimen (5 feet long) examined 

 was more rudimentary and less apparent than in the very 

 young specimen about 4 inches lone;, Avhich is figured in 

 PI. VL fig. 5. 



The animal, unlike the generality of Chelonians, appears to 

 be chiefly supported by its hard, longitudinally costate skin. 

 The skin is very thick, and the whole outer surface is studded 

 with very close hard hexangular disks, more like the surface 

 of a trunkfish [Ostracion) than any thing that I can compare 

 it with. These disks are larger and more oblong on the 



