STRUCTURE OF TORTOISES. 29 



it, the same pieces which compose the skeleton of the higher 

 vertebrate animals; except that many of these pieces are 

 changed in form and size. 



4. When we examine the carapax of a Tortoise, (Fig. 4,) 

 we see that it is formed of a considerable number of bony 

 plates, united to each other by sutures, and that 



eight of these plates occupy the middle line, on 

 each side of which, sixteen form a longitudinal 

 row, and twenty-five or twenty-six surround 

 the whole, like an oval frame It is then difficult 

 to recognise the nature of these bones ; but, 

 if \ve examine the carapax on its inferior sur- 

 face, \\e at once see (Plate l,./fy. 3.; that the 

 middle pl?ces are simply dependencies of the 

 dorsal vertebrae, (vd.) In fact, we find be- 

 neath it, the body jf each one of these bones of 

 its ordinary form, as well as the vertebral canal' 

 for the lodgement of the spinal marrow ; but the superior portion 

 of the parietes of the ring, which constitutes this canal, instead of 

 having, as is usual, the form of a transverse bony band, separated 

 by a space from its fellows, and being surmounted by a spinous 

 process, is here widened like a disk, and without any interrup- 

 tion is continuous with the analogous plates belonging to the 

 vertebra which precedes, and that which follows it. Each of 

 these dorsal vertebrae, in this way rendered immoveable, sustain 

 a pair of ribs as in man, and other vertebrata ; but these ribs 

 are widened so as to touch each other throughout, or nearly 

 throughout their whole length, and join each other by sutures (c.) ; 

 the marginal pieces, which articulate with the extremity of the 

 ribs, and in a degree surround the carapax, evidently represent 

 the sternal portion of these bones, which, in mammals, remain 

 always in a cartilaginous state; but in birds, they are completely 

 ossified. They also remain cartilaginous in some Tortoises, and 

 many of them laterally support themselves by the edges of the 

 sternal plastrum. 



5. The sternum of Tortoises presents an extraordinary 

 developement ; it extends Irom the base of the neck to the 

 origin of the tail, and covers the whole inferior surface oj 

 the body, (Fig. 5.) The pieces which enter into its compo* 

 sition, are nine in number, and, instead of being placed in 

 a row as in mammals, with a single exception, they are 

 arranged in pairs, and soldered or articulated with each other, 



4. What is the Carapax ? How is it formed ? 



5. What is the Plastrum? HQW is jt constituted ? 



2* 



