6 GUIDE TO EEPTILES AND AMPHIBIANS. 



to the margins of the jaws, the two-headed ribs, the absence of a 

 perforation in the lower end of the humerus, and the adaptation of 

 the limbs for walking. 



They differ in that the ischia, or anterior elements of the lower 

 part of the pelvis, unite in the middle line of the abdomen, and by 

 the circumstance that when the vertebrae articulate by cup-and-ball 

 joints, the cup (except occasionally in the tail) is behind (opistho- 

 coelous). No Dinosaurs have the pitted bony plates found in most 

 Crocodiles. 



The group is divided into four sub-orders : — 



I. Sauropoda. — Includes gigantic herbivorous, plantigrade Rep- 

 tiles, walking on all four limbs, with teeth in the front of 

 both jaws, the pubes of the pelvis simple and meeting in the 

 middle line of the abdomen, and the trunk-vertebrae with 

 lateral cavities. The teeth are spatulate, with smooth edges. 

 Some of the species, like Brontosaurus, were about sixty feet 

 in length and ten in height. Generally, as in Cardiodon 

 (Cetiosaurus) and Diplodocus (41), the skull is small. 

 II. Theropoda. — The members of this group differ from the 

 Sauropoda by their digitigrade feet, carnivorous habits, 

 laterally-compressed and serrated teeth, and the absence of 

 excavations in the trunk-vertebrae. Many of them, like 

 Ifegalosaurus (39) and the diminutive Compsognathus, 

 assumed the erect posture. 



III. Orntthopoda. — The division of the pubis into a pre-pubic and 



a post-pubic branch, neither of winch meets in the middle 

 line of the abdomen, forms a distinctive feature of this 

 group, in which the front of both jaws is devoid of teeth, 

 while the lower jaw is provided with a distinct premandibular 

 bone. Teeth complicated, seldom in separate sockets. All 

 the forms are herbivorous. In one section (Stegosauria) the 

 feet are plantigrade, with more than three toes, the limb- 

 bones are solid, and bony plates and spines protect the body. 

 Scelidosaums (42), Stegosaurns, and Hylwosaurus are well- 

 known genera. In a second section (Iguanodontia) the 

 hind-feet are digitigrade, with three functional toes, the limb- 

 bones hollow, and the body unarmoured. The group includes 

 Iguanodon (43), Gamptosaurus, Trachodon, etc., all bipedal. 



IV. Ceratopsia. — Includes gigantic quadrupedal Reptiles, with a 



bony neck-shield, a premandibular and a prerostral bone, a 



