384 



DIXOSAURIA. 



the tail which might conceivably have served as a swimming organ. How- 

 ever this may be the great number of them which have been found as 

 fossils seems to indicate that they lived in swamps or in the neighbourhood 

 of water, that is to say in places where their bodies would after death bo 

 rapidly covered by sediment. They have left their footprints in the 

 sandstone (Triassic) of the Connecticut valley, and other parts of N. 

 America. By their skull and one or two other features the Dinosauria pre- 

 sent resemblances to Crocodilia and Rhynchocephalia, but in many of them 

 the shoulder girdle, pelvis and hind limb are strongly avine in character. 



Order 1. THEBOPODA. 



^-TjDigitigrade carnivorous Dinosaurs with cutting teeth, and small sk\ill 

 set at a right angle with the neck. The cranium is incompletely ossified 



Flo. 210. Anchisaurus colurus, skull, A from the side, B from above, C from behind (from 

 Woodward, after Marsh), x \. Trias, Connecticut, a external nostril ; b preorbital vacuity ; 

 bp basipterygoid vacuity ; c'lateral temporal fossa, d supratemporal fossa ; / frontal ; j jugal ; 

 n nasal ; o orbit ; oc occiptal condyle ; p parietal ; p' paroccipital process ; pj prefrontal 

 pm premaxilla ; q quadrate. 



and there is a large preorbital vacuity (Fig. 210). The vertebrae are some- 

 times and the limb bones are always hollow. The fore-limbs are smaller 

 than the hind-limbs and the progression was probaby mainly bipedal. 

 Both pubis and ischium meet in a ventral symphysis and there is no post- 

 pubic process. The digits are from three to five and have prehensile 

 claws. The astragalus sends up a process which is firmly fixed to the front 

 side of the tibia. They vary much in size. Compsognathus the smallest 

 Dinosaur was no larger than a cat, while Megalosaurus attained the 

 dimensions of an elephant 



They extend from the Trias to the Cretaceous. They are the earliest 

 Dinosaurs as yet known and are usually regarded as being the most 

 generalised of the group. 



