THE CAKAPACE OF THE CHELONIA. 199 



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 upwards to unite suturally 

 with the neural plate ; and, in front and behind, becomes 

 similarly connected with preceding and succeeding costal 

 flates. The rib may be traced along the under-surface 

 of the costal plate, beyond the outer margin of which it 

 protrudes ; and its free extremity is received into a pit in 

 an elongated prismatic dermal ossification, which forms one 

 of a series of marginal plates (Fig. 62. M). 



The first dorsal vertebra difi'ers from the others in many 

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

 looks downwards and forwards, while its prezygapophyses 



Fisr. 62. 



fig (j2. Transverse section of the skeleton of Chelone midus in tlie 



dorsal region :—C', centrum; V, expanded neural plate; C, costal 

 plate; R, rib; M, marginal plate; P, lateral element of the 

 plastron. 



are much prolonged, in order to articulate with the convex 

 posterior face of the centrum and prolonged postzygapo- 

 physes of the last cervical vertebra. The spinous process 

 of this vertebra does not pass into the bony nuchal plate of 

 the carapace, which lies above it (Fig. 63, Nu), 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 vertebra is shorter, from before back- 

 wards, than its centrum ; and the neural arch of the second 

 dorsal vertebra extends forwards and overlaps the centrum 

 of the first, for the space thus left unoccupied. The rib of 

 the second vertebra is also carried forward, and articulates 



