EXTIN-CT ORDERS OF. REPTILES. 451 



Mammals on the other. Most of the Dinosauria were of 

 gigantic size, and the order is defined by the following 

 characters : — 



The skin was sometimes naked, sometimes furnished with a 

 wellrdeveloped exoskeleton,- consisting of bony shields, much 

 resembling those of the Crocodiles. A few of the anterior ver- 

 tebrae, were opisthocoelous, the remainder having flat or slightly 

 biconcave bodies. The anterior trunk-ribs were double-headed. 

 The teeth were confined to the jaws and implanted in distinct 

 sockets. There were always two pairs of limbs, and these were 

 strong, furnished with claws, and adapted for terrestrial pro- 

 gression. In some cases the fore-limbs were very small in 

 proportion to the size of the hind-limbs. No clavicles have 

 been discovered. 



The most familiar examples of the Dinosauria are Megalo- 

 saurus and Iguanodon. 



Megalosaurus is a gigantic Oolitic Reptile, which occurs also 

 in the Cretaceous series (Weald Clay). Its length has been 

 estimated at between forty and fifty feet, the femur and tibia 

 each measuring about three feet in length. As the head of 

 the femur is set on nearly at right angles with the shaft, whilst 

 all the long bones contain large medullary cavities, there can 

 be no doubt but that Megalosaurus was terrestrial in its habits. 

 That is was carnivorous and destructive in the highest degree is 

 shown by the powerful, pointed, and trenchant teeth. 



The Iguanodon is mainly, if not exclusively, Cretaceous, 

 being especially characteristic of the great delta-deposit of the 

 Wealden. The length of the Iguanodon has been estimated as 

 being probably from fifty to sixty feet, and from the close re- 

 semblance of its teeth to those of the living Iguanas, there is 

 little doubt that it was herbivorous and not carnivorous. The 

 femur of a large Iguanodon measures from four to five feet in 

 length, with a circumference of twenty-two inches in its smallest 

 part. From the disproportionately small size of the fore-hmbs, 

 and from the occurrence of pairs of gigantic three-toed foot- 

 steps in the same beds, it has been concluded, with much pro- 

 bability, that Iguanodon, in spite of its enormous bulk, must 

 have walked temporarily or permanently upon its hind-legs, 

 thus coming to present a most marked and striking affinity to 

 the Birds. 



The most remarkable, however, of the Dinosauria, is the 

 little Compsognathus longipes from the Lithographic Slate of 

 Solenhofen, referred to this order by Professor Huxley. This 

 Reptile is not remarkable for its size, which does not seem to 

 have been much more than two feet, but for the remarkable 



