ORDER CHELONIA. 



095 



suprapygal bone, and some (fig. 10 14) or occasionally all of the 

 neural bones are absent ; and when epidermal shields are developed 

 an intergular (as in fig. 10 12) is present on the plastron. This 

 shield is, however, sometimes found in the next section, in which 

 the neurals may also be reduced in number. The entoplastral 

 bone (as in the preceding section) is either oval or rhomboidal ; 

 and a mesoplastral bone (compare fig. 10 12) may be present. 



In the skull of existing and Tertiary types the tympanic cavity is 

 completely surrounded by the quadrate, which forms an unbroken 

 ring; the articular surface of the quad- 

 rate has a concavity for the reception of 

 a condyle on the mandible ; the pteryg- 

 oids are very broad and wing-like ; the 

 vomer may be absent, so that the pala- 

 tines may meet in the middle line ; and 

 there may be distinct nasals, and a suture 

 in the mandibular symphysis. The cer- 

 vical vertebrae have well-developed trans- 

 verse processes, and single terminal ar- 

 ticulations ; while there are never more 

 than three phalangeals to the digits. 



The Pleurodira are further character- 

 ised by their inability to retract the head 

 directly within the carapace ; but the 

 neck is bent on one side and the head 

 thus brought within the margin of the 

 shell. In all cases the labyrinth of the 

 ear is completely open from behind. 



At the present day this section is al- 

 most exclusively confined to the southern 

 hemisphere ; but in the Eocene forms 

 more or less nearly allied to existing 

 types were widely spread over the north- 

 ern hemisphere, and in the Mesozoic 

 there were many European representa- 

 tives of the group. With the exception 



of Podocnemis, the existing forms are carnivorous, and the whole of 

 them are thoroughly aquatic. 



Fig. 1014. — The right half of the 

 carapace of Sternothccrus nigri- 

 cans; from Madagascar. The thick 

 lines indicate the boundaries of the 

 epidermal shields. 



At least one of the two earliest known genera of Testudinata has been 

 referred to this section ; and from the occurrence in many existing forms 

 of separate nasals, and of a mesoplastral bone, we may regard this sec- 

 tion as retaining evidence of a close alliance with a primitive generalised 

 type which has been lost among existing Cryptodira, since it is nearly 

 certain that features like these if once lost would not reappear, except 

 as abnormalities. The presence of transverse processes to the cervical 



vol. 11. O 



