388 CHELONTA 



CHAP. 



G 



astonishing variability in the number of the horny shields of 

 the carapace. The normal number of shields of the carapace, 

 leaving out the marginals and counting the nuchal as the first 

 neural, is 6 neurals and 5 pairs of costals, in all 16. The 

 greatest number of dorsal shields observed is 8 neurals and 8 

 pairs of costals, in all 24. Many of the intermediate combina- 

 tions have been observed, there being, for instance, specimens 

 with 8 neurals and 16, 14, 13, 12, or 11 costals, the latter not 

 being always in pairs, but unequal on the right and left sides ; 

 or there are 7 neurals with 20 to 16 costals, or 6 neurals with 

 20, 19, 18, 17, or 16 costals. The interesting fact in con- 

 nexion with these variations is, moreover, that some of the shields 

 are much smaller than the others, sometimes mere vestiges in 

 all stages of gradual suppression, and that the abnormalities are 

 much more common in babies and small specimens than in 

 adults. The importance of these " orthogenetic " variations has 

 been discussed on p. 326. 



Sub-Order 2. Pleurodira. — Neck bending laterally and tuched 

 away in the niche formed between tlie anterior jiortion of the cara- 

 Ijace and plastron. Pelvis anhylosed to the shell, the broadened 

 tops of the ilia to the carapace, the distal ends of the ^lubes and, 

 ischia to the plastron. 



Freshwater tortoises, almost entirely carnivorous, inhabiting 

 South America, Australia, Africa, and Madagascar. Fossil forms 

 are known from the Jurassic epoch onwards. 



Owing to the strong connexion of the iliac bones with the 

 costal plates the sacrum has become practically abolished, the 

 sacral ribs being reduced to one pair (the posterior of the 

 original two pairs) or being absent. The centra of the cervical 

 vertebra articulate by cup and ball joints. The formation of 

 the temporal region of tlie skull varies considerably in the three 

 families, some genera lacking the complete zygomatic arch, while 

 others have a narrow parieto-squamosal arch bridging over the 

 temporal fossa, or the latter is completely roofed over by the 

 laterally expanded parietal, which meets the jugal and (piadrato- 

 jugal. The quadrate is always trumpet-shaped ; the rim of the 

 tympanum is complete, but the posterior part of the trumpet 

 remains open. The basisphenoid, pterygoids, and palatines form 

 a broad and Hat roof to the mouth. The vomer is large, and 

 separates the palatines in the Chelydidae ; it is very much 



