LIFE IN SECONDARY TIMES 267 



approach ten metres in length. These Stegosaurians were 

 strange beasts. The disproportion between their anterior and 

 posterior limbs caused considerable convexity in the back 

 when they went on all four feet, and along the line of this curve 

 in Scelidosaurus was arranged a double row of projecting 

 bony plates, which became unique on the greatly elongated tail. 

 The dorsal plates of the Stegosaurians formed but a single 

 row, but they were of great size, triangular, and erected 

 vertically, with the apices in the air, the largest of them being 

 almost a metre in height. This row of plates divided into a 

 double series on the tail, which thus carried two rows of spines 

 sixty centimetres in length. 



The digitigrade type became highly accentuated and con- 

 stant in the Ornithopods, which habitually held themselves 

 erect on their hind-quarters. Their long bones were hollow 

 and in communication with the air sacs like those of Birds. 

 We have already met with this character in the Theropods, 

 which had also adopted the biped attitude. This attitude 

 implies a greater expenditure of muscular effort in the posterior 

 body-region than the quadruped attitude. On the other 

 hand, the development of the air sacs adds considerably to the 

 respiratory power of the lungs. In the case of both Theropods 

 and Orthopods it is not impossible that the possibility of 

 acquiring their new method of locomotion, endowing them 

 particularly with an aptitude for running and leaping, was 

 due to this increase of respiratory activity. We find in 

 Ornithopods a series of forms analogous to those of the 

 Theropods. Camptosaurus of the Lower Jurassic and of the 

 lowest layers of the English Cretaceous (Wealden) had five 

 fingers and four toes. Hypsilophodon from the same localities 

 had only four fingers and four toes. The northern Ignanodon, 

 of the Neocomian of Belgium and Germany, had only four 

 digits on its fore-legs and three on its hinder, the pollex having 

 been transformed into a formidable spur. Hadrosaurns and 

 Trachodon of the same regions closely resembled them, except 

 that the mouth was prolonged into a sort of edentate duck-bill. 

 At the back of this bill were several rows of small teeth, forming 

 one functional row, followed interiorally by numerous rows 

 of replacing teeth, the total number being two thousand. 



The best known of all these Reptiles are the Iguanodons, 

 so named because their herbivorous teeth resembled those of 



