58 VERTEBRATA. 



No. 88. [276] Saurophidium Thollieri. 



Skeleton, on slab (cast). 

 From the lithographic lime- 

 stone in the Department of Ain, 

 France, and now in the Museum 

 of Natural History at Lyons. 

 Size, 2 ft. 7 in. x 11 in. 



ORDER CHELONIA. 



This is tlie most circumscribed order of reptiles, and most 

 easily distinguished by the solid immovable armor encasing the 

 greater part of the body, and the toothless, horny beak. The 

 scapular and pelvic arches are within the bony box formed of the 

 united vertebrae and ribs, which causes the scapula to be singu- 

 larly placed inside the ribs. 



The dorsal shield or carapace is least complete in marine tur- 

 tles, which fact affords an important aid in the discrimination of 

 fossil Chelonians. Excepting in the Soft or Mud-Tortoises, the 

 whole armor is covered with dermal plates or ossified skin ("tor- 

 toise-shell") homologues to the scutes of the crocodile. The 

 lower jaw is one solid arch. The only movable vertebrae are the 

 cervical and caudal. Some of the cervicals are convexo-concave, 

 others concavo-convex, one biconcave (usually the 8tli). The 

 caudals are procoelian. The plastron is broad in the land species, 

 narrow in the marine. They all possess limbs, and the land species 

 furnish the first instance of real walking in the vertebrate series, 

 unless the running of some toads be considered as such, for 

 Salamanders, Lizards and Crocodiles depend partly on the wrig- 

 gling of the spinal column. The feet of the marine Chelonians 

 are fin-shaped ; of the fluviatile and marsh species, palmated ; of 

 the land species, club-shaped. Over two hundred species are 

 known, the fresh-water forms being most numerous. 



The two shields, usually in fragments, are the chief evidence 

 of extinct Chelonians. The beaks are sometimes found solitary, 

 in the Chalk. 



Fossil forms are abundant in American Cretaceous and Eocene, 

 some of great size, and a species of Compsemys occurs in Upper 

 Jurassic. They occur at a lower horizon in Europe. 



