CHELONIA. 173 



typical species is P. lullocki, having a shell sometimes about 0'5 m. in 

 length. P. portlandicum, from the Portland Beds of the Isle of Portland 

 (tig. 108), is smaller. 



Fio. 109. 



Pleurosternum bullocki; visceral aspect of posterior end of plastron, about one- 

 quarter nat. size. U. Jurassic (Purbeck Beds) ; Dorsetshire. /, facette for 

 pubis; hp.p., hypoplastron ; xp., xiphiplastron. (After Lydekker.) 



Sub-Order 1. Grygtodira. 



The majority of existing Chelonia belong to that section in 

 which the head is retracted by curvature of the neck in a 

 vertical plane, while the pelvis is unconnected with the 

 plastron. Most known extinct types may also be placed in 

 the same group. 



Typical marine turtles first occur in the Upper Cretaceous 

 of western Europe, one large form (Chelone hofma.nni) being 

 met with in the yellow chalk of Maastricht, Holland, other 

 fragments and a diminutive shell (Chelone benstedi) being 

 known also from the English Chalk. The first evidence of 

 turtles with an extremely degenerate shell has also been dis- 

 covered in the Upper Cretaceous of Italy, and of Kansas and 

 Dakota, U.S.A. The Italian fossil, Protosphargis veronensis, is 

 a shell measuring nearly three metres in length. Its costal 

 bones are slightly more developed than in the surviving 

 "leathery turtle" (Sphargis or Dermochelys}, but nothing is 

 known of any outer dermal armour. Its marginal bones are 

 very slender. The plastron lacks the entoplastron (inter- 

 clavicle). The best-preserved American fossil, Protostega gigas, 

 comprises part of the skull, some crushed vertebras, and frag- 

 mentary remains both of the shell and pectoral limbs. It is as 



