REPTILIA. 191 



sively confined to rocks of this age. In the Tertiary period, 

 lastly, the remains of Reptiles are comparatively rare, and 

 the number of types is much reduced. The living order 

 of the Opliidia, however, makes its first appearance in the 

 Tertiary deposits. In the following view of the characters 

 and distribution in time of the orders of the Reptiles, it will 

 be advisable to consider the recent orders first, though this 

 is not in accordance with their natural arrangement. 



Order I. Chelonia. — The first order of living Reptiles is 

 that of the Chelonia, comprising the Tortoises and Turtles, 

 and distinguished by the following characters : There is an 

 osseous exoskeleton ivhich is combined loith the endoskeleton to 

 form a kind of bony case or box in which the body of the 

 animal is enclosed, and which is covered by a leathery skin, or, 

 more usually, by horny epidermic plates. The dorsal vertehi-ce 

 are immovably connected together, and are devoid of transverse 

 processes. The ribs are greatly expanded {fig. 547, r), and 

 are united to one another by sutures, so that the vmUs of the 

 tlioracic cavity are immovable. All the bones of the skull 

 except the lower jaw and the hyoid bone are immovably tmited 

 together. There are no teeth, and the jaws are incased in horn 

 so as to form a kind of beak. The heart is three-chambered, 

 the ventrictdar septum being imperfect. 



Of these characters of the Chelonia, the most important 

 and distinctive are the nature of the jaws and the structure 

 of the exoskeleton and skeleton. As regards the first of 

 these points, the lower jaw in the adult appears to consist 

 of a single piece, its complex character being masked by 

 anchylosis. The separate pieces which really compose each 

 ramus of the jaw are immovably anchylosed together, and 

 the two rami are also united in front by a true bony union. 

 There are also no teeth, and the edges of the jaws are 

 simply sheathed in horn, constituting a sharp beak. As 

 regards the second of these points, the bony case in which 

 the body of a Chelonian is enclosed consists essentially of 

 two pieces, a superior or dorsal piece, generally convex, 

 called the " carapace," and an inferior or ventral piece, 

 generally fiat or concave, called the " plastron." The cara- 

 pace and plastron are firmly united along their edges, but 



