172 THE ANATOMY OF VERTEBRATED ANIMALS. 



fclie first dorsal vertebra, upon whicli tlie former play back- 

 ward and forward. 



All tlie cervical vertebrae are very freely movable upon 

 one another, and confer great flexibility on the neck. In 

 strildng contrast with this arrangement, the ten following 

 vertebrte have flattened faces, firmly united by cartilage. If 

 any one of these vertebras, from the second to the ninth, be 

 examined, it will be found that the elongated centrum is only 

 loosely united with the neural arch, and that the summit of 

 the neural arch is continuous with a broad flat plate of bone, 

 which forms one of the eight median elements of the carapace, 

 or neural plates (Fig. 62, Y). 



There are no transverse processes, but a rib is articulated 

 between the centrum and the neural arch. At a short dis- 

 tance from its attachment, this rib passes into a broad plate 

 of bone, which extends upward to unite suturally with the 

 neural plate ; and, in front and behind, becomes similarly con- 

 nected with preceding and succeeding costal plates. The rib 

 may be traced along the under-surface of the costal plate, be- 

 yond the outer margin of which it protrudes ; and its free ex- 

 tremity is received into a pit in an elongated prismatic dermal 

 ossification, which forms one of a series of marginal plates 

 (Fig. 62, 31). 



The first dorsal vertebra differs from the others in many 

 respects. The anterior face of its centrum is concave, and 

 looks downward and forward, while its prezygapophyses are 



C --^r^F 



\ 



Fio. 63. — ^Transverse section of the skeleton of Clidone midas in the dorsal region : <?•, 

 centrum; V, oxpanded neural plate; C, costal plate; R, rib; M, marginal plate; P, lat 

 eral element of the plastron. 



much prolonged, in order to articulate with the convex poste- 

 rior face of the centrum and prolonged postzygapophyses of 

 the last cervical vertebra. The spinous process of this verte- 

 bra does not pass into the bony nuchal plate of the carapace, 

 which lies above it (Fig. 63, Nii)^ and its rib does not expand 

 into a costal plate, but becomes connected with the costal 

 plate of the second dorsal vertebra. The neural arch of this 



