DIXOSAURIA. 



387 



skull is laterally compressed, has a small orbit, and small preorbital 

 vacuities ; the premaxillae have a cutting edge and are edentulous ; the 

 maxillae and mandible bear the teeth which are often worn down to a 

 grinding surface ; the jugal is a crescentic bone below the orbit, and the 

 quadrate is elongated. There are about 80 vertebrae (10 cervical, 18 

 dorso-lumbar, 4-6 sacral and 40-50 caudal) ; all bear ribs except the atlas, 

 two or three lumbar, and the posterior caudal ; the neural spines of the 

 back and tail are very strong and frequently show traces of ossified ten- 

 dons. The caudals have chevrons. The scapula is long, the coracoid 

 small and there is a pair of sternal bones. Manus with 5 digits of which, 

 the pollex is a spur-like process ; digits 2 and 3 are tipped with hoof-like 

 nails. The ilia are greatly extended antero-posteriorly : there is an ischi- 

 adic symphysis, and a slender postpubic process. The femur has a 

 prominent inner (4th) trochanter and the pes has 3 digits with claw-shaped 

 terminal phalanges. Hypsilophodon Huxley, complete skeleton from the 



FIG. 213. Skeleton of Stegosaurus ungulatus (after Marsh, from Woodward) ; U. Jura, 



Colorado, x S V 



Isle of Wight. Laosaurus Marsh, U. Jura, Colorado ; Claosaurus Marsh, 

 U. Cretaceous, Wyoming ; Trachodon Leidy, U. Cretaceous, U.S.A. 



Tribe 2. Stegosauria. Armoured, quadrupedal plantigrade forms, 

 with solid bones and small skull. Lias to U. Cretaceous. Stegosaurus 

 Marsh (Fig. 213), head small ; brain minute, smaller than the large sacral 

 swelling of the spinal cord ; teeth numerous and small ; cervical vertebrae 

 with ribs, neural spines expanded to support the dermal armour ; sacrum 

 of 4 fused vertebrae ; anterior caudal vertebrae very large ; fore-limb short, 

 powerful, ulna with large olecranon process ; ilium extends far forward ; 

 astragalus and calcaneum united with the tibia and fibula whch are short ; 

 dermal armour of large triangular plates along the back and indications 

 of small rounded ossicles on the throat, to 28 feet, U. Jura of Colorado 

 and Wyoming. Scelidosaurus Owen, L. Lias, England. 



Tribe 3. Ceratopsia. Gigantic probably herbivorous, quadrupedal 

 reptiles with a large skull, which carries a pair of horn-like processes in the 

 frontal region just above the orbit and an unpaired process in the nasal 



