DINOSAURIA. 205 



fur the extension of the maxilla) (m) and premaxillse (pm.\ the length of 

 the basipterygoid processes, the great extent of the pterygoids (pt.), and 

 the very small development of the votners (v). The successional teeth 

 are beautifully shown in a cavity of the bone both in the upper and lower 

 jaws (tig. 124). The chevron bones of the tail are unique in being double, 

 each having both anterior and posterior branches (hence the generic 

 name). The sternal bones have been discovered as in Brontosaurus (fig. 

 122, p. 202). The genus is known only from the Jurassic of North 

 America, and the typical species, Diplodocus longus, must have attained 

 a length of about twelve metres. The position of the external nares 

 suggests aquatic habits. 



Sub-Order 3. Ornithopoda (or Predentata). 



These are also herbivorous Dinosaurs of very varied pro- 

 portions, ranging throughout Jurassic and Cretaceous strata. 

 Some must have been bipedal in gait, others quadrupedal ; but 

 all agree in the characters of the pelvis and the presence of a 

 predentary bone in front of the mandible. The brain-case is 

 completely ossified, and there is little or no antorbital vacuity 

 in the skull. The cervical ribs loosely articulate with their 

 respective vertebrae. The pubis is slender and directed down- 

 wards and forwards, but does not meet its fellow in the median 

 line in front ; at its base there arises a slender post-pubis which 

 is directed backwards and downwards parallel with the equally 

 slender ischium. The astragalus does not exhibit an ascending 

 process. 



The typical unarmoured Ornithopoda must have been 

 bipedal in gait and digitigrade. They are represented by the 

 three-toed Iguanodon and the four- toed Hypsilophodon in the 

 European Wealden ; by Camptosaurus and Laosaurus in the 

 Jurassic, Hadrosaurus and Claosaurus in the Cretaceous of 

 North America. 



Iguanodon (figs. 125127). This is one of the first-discovered Dino- 

 saurs, thus named from the resemblance of its teeth to those of the modern 

 lizard, Iguana.' Complete skeletons are known from the Wealden of 

 Bernissart, near Mons, Belgium. The only skulls hitherto found are very 

 much fractured, though exhibiting the limits of most of the elements. 

 The head (fig. 126) is laterally compressed, the nostrils are relatively 

 large, nearly terminal, and there is a very small antorbital vacuity. The 

 frontals are flattened and apparently fused together, the parietals meet in 



