170 REPTILIA. 



Zealand. Polyptychodon is a European Cretaceous representa- 

 tive of Pliosaurus known only by teeth and fragments. 



ORDER 3. CHELONIA. 



Palaeontology as yet affords no information concerning the 

 origin of the tortoises, turtles, and their allies, and very little as 

 to the evolution of the order they represent. The skull in the 

 Triassic Dicynodonts, as already remarked, is curiously Chelonian 

 in aspect. The armoured Permian Pariasaurian, Otoccelus 

 (p. 149) is also suggestive of a possible ancestral Chelonian 

 type. The scapula in Plesiosaurians exhibits a great ventral 

 production much resembling the same feature in Chelonians ; 

 and it is conceivable that the very highly developed system of 

 abdominal ribs in the order just mentioned might admit of 

 modification into a Chelonian plastron. Other characters, such 

 as the conical epiphyses of certain limb-bones, also seem to imply 

 community of origin of the Chelonia with the Sauropterygia 

 and Batrachia. The earliest known remains of a Chelonian 

 shell, however, from the Upper Trias of Germany, are as typical 

 of the order as the corresponding parts of a modern tortoise. It 

 is only to be regretted that no satisfactory evidence of the head, 

 neck, and limbs has hitherto been discovered below the Upper 

 Jurassic. 



The earliest known Chelonian remains, from the Upper 

 Keuper of Wtirtemberg, comprise a natural sandstone mould of 

 the interior of a shell (Proganochelys quenstedti or Psammochelys 

 keuperina) in the Tubingen Museum, and various fragments of 

 a carapace (Chelytherium obscurum) in the British Museum. 

 The first shows that the costal plates are present to the normal 

 Chelonian number of eight pairs, while the only vacuities in the 

 shell are a series of small openings along the junction between 

 the carapace and plastron on each side. The remains of Chely- 

 therium indicate that each dorsal rib was single-headed, articu- 

 lating at the line of contact between two vertebral centra in the 

 ordinary manner ; and 'deep sulci on the outer aspect of the 

 bones of the carapace evidently denote the boundaries of 

 epidermal shields present during the life of the animal. 



