CiiAi>. II. THE CHELYOID.E. 337 



The curve from side to side of the outer surface of the carapace is interrupted 

 by three ridges, formed by the increased thickness of the derm, besides the 

 depressions spoken of al)Ove, which enter into the form of the body itself The 

 middle ridge passes along over the vertebral column ; it is slight at the front 

 end of the shield, broadest above the first two or three dorsal vertebrae, higher 

 and narrower backward above the sacrum, and then decreases to the hind end 

 of the shield ; it occupies the space between the <lepressions already mentioned. 

 The other ridges are smaller, and situated just outside of these depressions. 



The shield is thick, completely ossified, and regularly divided into plates. Be- 

 sides the eleven pairs of marginal plates and the eight pairs of costals, the usual 

 plates of the vertebral row, with the odd plates at each end terminating the mar- 

 ginal rim, are constant in the carapace. The odd plate and the other marginal 

 plates in front, as well as the first pair of costals, are very large, and give the 

 unusual length and breadth to the carapace in front of the first costal arch. 



The plastron is made up of nine plates, as usually, foiu* pairs and one odd 

 one. The second and third pairs reach entirely across, unite with the carapace 

 on each side, and form the bridges and the greater part of the flattened por- 

 tion of the plastron. The first pair meet in front, and are united by a bony 

 suture, and, reaching backward more than outward, arc joined to the second pair 

 by sutures of about the same length. These and the odd plate are large, and 

 give the unusual size to the front part of the plastron. The fourth pair is the 

 smallest, and just vmderlies the pelvis. 



The scapular arch, down to the shoulder joints, is nearly perpendicular. The 

 iliac bones are nearly perpendicular and parallel ; their upper ends are very 

 large, and are firmly sutured to the shield above. The ischium too is sutured 

 to the shield below, as also is the pubis. Thus the pelvis is firmly fixed to the 

 shield above and below. This support, together with that of the strong bridges, 

 the thickness of the bony derm generally, and the additional ridges of the cara- 

 pace, make the shield ver}- firm, in spite of the rather slight curvature of the 

 carapace from side to side. 



The ribs extend far out from the vertebrce before meeting the shield, and 

 the space above them on either side of the spinous apophyses is wide as well 

 as high, and affords place for the passage and attachment of very large muscles. 



The first dorsal vertebra is turned down at the front end. and its body is 

 much enlarged, so as to present a large, round, articulating surface. Its articidating 

 processes, instead of reaching as usualh' outward and downward, are placed higher 

 up, near together, and make, with the body of the vertebra, a long, perpendic- 

 ular axis, upon which the adjoining neck vertebra swings freely from side to side, 

 and but little up and down. This is the prevailing direction of the axis through 



43 



