304 TEETIAET VEETEBEATA OF THE PATtM. 



owing to the suppression of the anterior and posterior faces respectively ; in the 

 anterior neurals the an tero -lateral borders are much shorter than the postero-lateral ones. 



There are eight pairs of costals : as already mentioned, the first and last pairs meet 

 in the middle line (text-fig. 97, A). The nuchal [Wu.) is large and wider than long ; 

 it is wider than the anterior vertebral plate which overlaps it; the anterior border is 

 emarginate. There are eleven pairs of marginals, of which 4, 5, 6, and 7 form the 

 base of the bridge, and 5 and 6 unite with the mesoplastial (Ms.p.). The anterior 

 free marginals have a rounded edge, the posterior ones are somewhat expanded and 

 have a sharp edge. The axillary buttress is opposite the fourth marginal, the inguinal 

 opposite the seventh : both buttresses are weaker and less developed than in most 

 Pleurodirans, the inguinal being the stronger of the two. 



The suprapygal {Pyg.) occupies the summit of the posterior prominence above referred 

 to ; it is roughly triangular in outline with convex sides and is of nearly the same 

 shape as the overlying vertebral shield. The pygal is notched in the middle line 

 by the furrow separating the posterior pair of marginal shields. 



There are five vertebral shields [v.), the anterior of which is very narrow, being much 

 narrower than the nuchal bone beneath it ; in Pelomediisa and Podocnemis the reverse is 

 the case. The remaining vertebrals are roughly hexagonal in form, the antero-lateral 

 borders being slightly shorter than the postero-lateral. The posterior shield is very 

 convex ; like the underlying suprapygal it is triangular in form, but is much larger 

 (text-fig. 97, A, C). There are four costal shields and twelve marginals: no nuchal 

 shield is present. On the anterior borders of the carapace the extent to which the 

 marginals are exposed is very small, but posteriorly, where the carapace is somewhat 

 expanded, the area is much greater. The relations of the shields to the underlying 

 bones are shown in the figure. 



The 'plastron (text-fig. 97, B) is large; its posterior free portion is wider than the 

 anterior and is more than half as long again. The length of the bridge is about equal 

 to the width of the front lobe, but much greater than its length. The anterior border 

 is gently concave. The upper surface in this region is also gently concave from side 

 to side, the concavity being bounded by thickened ridges borne on the epiplastrals 

 and anterior part of the hyoplastrals [Hy.p.]. From this ridge the surface slopes 

 sently away to the lateral border of the anterior lobe. It appears that the upper 

 surface of the anterior lobe of the plastron was covered for some distance from its 

 border by a series of epiplastral shields. 



The entoplastron {Ent.) is a small escutcheon-shaped element, from the outer angles 

 of which the sutures between the epiplastral and hyoplastral bones run forwards, 

 making an angle of about 45° with the long axis of the shell and terminating on the 

 border of the plastron in the notch marking the end of the. groove between the 

 humeral and pectoral shields. The suture between the hyo- and hypoplastra [Hy.p., 

 Hyp.p.) crosses at the level of the middle of the bridge and terminates externally 



