314 PALEONTOLOGY. 



the colubrine, and venomous families, at early tertiary periods, 

 before any of the existing species of mammalia had appeared 

 on the earth. The eocene and miocene fossils demonstrate, 

 moreover, the same adaptation to a prone posture and a glid- 

 ing movement with the belly in the dust, as at the present 

 day ; and the fossil vertebras exhibit the same peculiar com- 

 plexities which so exquisitely adjust the vertebral column of 

 the serpent to do the work of hands, feet, and fins ; — to out- 

 climb the ape, outswim the fish, outleap the jerboa, and 

 outwrestle the tiger. 



Order XII. — Chelonia. 

 {Tortoises and Turtles) 



Char. — Trunk-ribs broad, flat, suturally united, forming, with 

 their vertebrae, the sternum, and dermal bones, an ex- 

 panded thoracic-abdominal case, into which the limbs, 

 tail, and, usually, the head, can be withdrawn. No 

 teeth : external nostril single. 



The most common evidences of extinct chelonians are 

 the fossil remains of the above-defined case, usually in frag- 

 ments or detached portions ; and, as this natural and portable 

 dwelling-chamber offers modifications characteristic of the 

 chief divisions of the order, some guide to the knowledge or 

 study of its composition is here premised. 



In the marine families called turtles (Chelone), and mud- 

 turtles (Trionyx), it consists of a floor or " plastron" (fig. 108, b), 

 and a roof or " carapace" (ib. a). Side-walls are added in the 

 fresh-water terrapenes (Emys), and land-tortoises (Testudo). 

 The carapace is composed of a series of medial and symmetrical 

 pieces, ch to py 9 and of two series of unsymmetrical pieces on 

 each side. The medial pieces, called "neural plates," are 

 dermal bones, of which those marked s 1 to s 8 are connate 

 with the summits of the spines of as many dorsal vertebra?, 



