EXTINCT ORDERS OF REPTILES. 231 



also remain distinct, and are not anchylosed with any of tlie 

 tarsal bones to form a " tarso-metatarsus." 



The Deinosaiiria are exclusively Mesozoic, ranging from 

 the Triassic to the Cretaceous formation, but abounding 

 especially in the Oolitic and the earlier portion of the Cre- 

 taceous period. By Professor Huxley the " Thecodont " 

 Eeptiles are regarded as belonging here, as has been already 

 remarked. 



The number of known Deinosaurian Eeptiles is already 

 extremely large, but as many are only very imperfectly 

 understood, it will be sufficient here to briefly notice a few of 

 the more important or more interesting types. The number 

 of genera known in the Trias (such as Teratosaurus, Amphi- 

 saurus, Glepsysaurus, Bathygnathus, &c.) is not very large, but 

 even at this early period the order seems to have had a very 

 wide distribution. 



In the Jurassic and Cretaceous periods the order under- 

 went an immense development, and is represented by numer- 

 ous genera, such as Iguanodon, Hypsilophodon, Laosaurtis, 

 Hadrosaurus, Cionodon, Foikilopleuron, Hylceosaurus, Pola- 

 canthits, Acanthopholis, Cetiosaurus, Titanosaurus, Megalosaurus, 

 Zcelaps, Compsognathus, Chondrosteosatcnis, &c. Of the above 

 the most important types are Iguanodon, Hylceosaurus, Me- 

 galosaurus, Cetiosaurus, Compsognathus, and Chondrosteosaurus. 



The Iguanodon is mainly, but not exclusively, Cretaceous, 

 being especially characteristic of the great delta-deposit of 

 the Wealden. The length of the Iguanodon has been esti- 

 mated as being probably from fifty to sixty feet, and from 

 the close resemblance 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-limbs, and from the occurrence oip)airs 

 of gigantic three-toed footsteps in the same beds, it has been 

 concluded, with much probability, that Iguanodon, in spite 

 of its enormous bulk, must have walked temporarily or per- 

 manently upon its hind-legs, thus coming to present a most 

 marked and striking affinity to the Birds. 



