338 PALEOZOIC TIME. 



Reptiles. These teeth, moreover, have the labyrinthine internal texture 

 of the teeth of Ganoids (p. 521) ; and hence they are called Labyrin- 



the Batrachoids, in having (1) double occipital condyles; (2) teeth on the vomer; (3) 

 short, if any, ribs; (4) usually large palatine openings: and they approach Saurians in 

 having (1) the teeth stout and conical, and set in sockets; (2) the body covered with 

 plates or scales; (3) the size sometimes very great. The teeth have the labyrinthine 

 arrangement of the dentine and cement that characterizes the Sauroid fishes among 

 Ganoids (see Fig. 521), and which is still continued in that group among the living- 

 Gars ; and hence the name Labyrinthodonts. 



The Ganocephala are supposed to be Labyrinthodonts, while approaching Ganoid 

 fishes in the sculptured bony plates which covered the head, and in some other char- 

 acters. — Ex., Archegosaurus and Apateon. 



II. True Reptiles. 



The skeleton in the true Reptiles has (1) but one occipital condyle below the foramen; 



(2) a series of ribs; (3) a covering of scales or plates, with rare exceptions. 

 The existing species, and part of the extinct, belong to three tribes : — 



1. Snakes, or Ophidians. — (1) Body without legs, with rare exceptions; (2) no 

 sternum; (3) eyes without lids; (4) no external ear. 



2. Saurians. — Body (1) without a carapax, and with a tail; and having (2) four 

 feet (rarely two, or none) ; (3) a sacrum corresponding to two united vertebrae, sometimes 

 more; (4) eyes with lids, or seldom without; (5) usually an external ear-opening. 



3. Turtles, or Chelonians. — Body having (1) a carapax, or shell, made of several 

 pieces firmly united ; (2) a very large sternum, forming the under surface of the body; 



(3) a horny beak, instead of teeth : (4) an external ear opening; (5) neck and limbs very 

 flexible. 



Saurians. — The Saurians vary in length from a few inches to fifty or more feet. 

 In some, the teeth are set in sockets, as in the Thecodont Saurians (so named from 0»jktj, 

 a case, and bSovs, tooth) and Crocodilians. In others (Pleurodonts), the teeth are im- 

 planted in a groove, the outer border of which projects more than the inner; in others 

 (Acrodonts), they are soldered firmly to the salient part of the jaw-bone. 



The prominent tribes are the following, beginning with the highest in rank : — 



1. Dinosaurs (5eu*os, terrible, and craiipos, lizard). — Reptiles of great size, all now 

 extinct, having some mammalian and many bird-like characteristics: (1) the long bones 

 have a medullary cavity; (2) the pelvic arch and the hind-feet are nearly as in Birds; 

 (3) the sacrum consists of at least four vertebrce, a mammalian feature; (4) the cervical 

 vertebras are convexo-concave, as in Mammals; (5) the lower jaw in some species has 

 lateral motion, for trituration. They include the Megalosaur (p 445), Hylceosaur, 

 Iguanodon, Hadrosaur, etc. 



2. Crocodilians, or Cuirassed Saurians. — Body having (1) a cuirass, made of bony 

 plates; (2) large, conical teeth, in sockets, in a single row; (3) one jugale; two 

 premaxillary bones; (4) sacrum formed in general of two vertebras; (5) heart with four 

 cavities; external nostrils at the extremity of the snout. The modern species have 

 concavo-convex vertebrae, — that is, the anterior face is concave and the posterior con- 

 vex; in others, of the Teleosaur group, including the extinct Teleosaurs, Hyposaurs, 

 etc., they are biconcave. 



8. Lacertians, or Scaly Saurians. — Body having (1) corneous scales; (2) the teeth 

 rarely in sockets; (3) no jugale; one ventricle; one premaxillary bone; (4) sacrum 

 consisting of two vertebra?, at the most. The Lizards, Iguanas, and Monitors are the 

 types of the tribe. 



A few extinct species characterized by small scales are Thecodonts, like the Croco- 

 diles, so that they stand apart from the Lacertians, and are intermediate between them 

 and Crocodilians. Such are the Thecodontosaur, Paleosaur and Proterosaur (Fig. 697, 

 p. 373), — among the earliest of true Reptiles, and the precursors of the Crocodiles and 

 Dinosaurs. 



