BIRD-LIKE GROUP. 



37 



OUTER AND LATERAL VIEWS OF A TOOTH OF 

 THE IGUANODON. 



bar of the pubis is inclined backwards, parallel to the ischium, while it has 

 a secondary plate projecting forwards. In this parallelism of the pubis and 

 ischium these dinosaurs resemble birds (see the figure in Vol. III. p. 290), and birds 

 alone ; and from this and other features it is pretty certain that the latter are 

 derived from reptiles more or less closely allied to this or the preceding group of 

 dinosaurs ; the resemblance in the one case being closest in the structure of the 

 pelvis, and in the other of the hind-limb. All the bird-like dinosaurs are further 

 characterised by the presence of a separate chin-bone (pd in the figure on p. 3) 

 at the extremity of the lower jaw; by the 

 absence of teeth from the front of both 

 jaws ; by the teeth themselves approximat- 

 ing more or less closely to the type of the 

 one here represented, and by being fre- 

 quently not implanted in distinct sockets ; 

 and likewise by the vertebrae being com- 

 pletely solid throughout. The typical 

 representatives of this group are the well- 

 known iguanodons, originally described on 

 the evidence of teeth, from the Wealden 

 rocks of England, but now known by 

 entire skeletons from the corresponding 

 deposits of Belgium, which are exhibited in 

 the museum at Brussels. These reptiles, 

 which were represented by allied forms in the United States, habitually walked 

 on their three-toed hind-limbs, the largest individuals attaining a length of some 

 33 feet. They are characterised by the limb-bones being hollow, by the length 

 of the metatarsal bones of the foot, by the first digit of the five-toed fore-limb 

 being converted into a large conical spine, and also by the teeth being of the type 

 of the one shown in the accompanying figure. Needless to say, animals with 



such teeth must have been purely vegetable 

 feeders, as indeed were all the other members 

 of this group. The hind - feet terminated in 

 rather sharp claws, and there was no bony 

 armour on the body. The iguanodons probably 

 stalked about among the palm -forests of the 

 Wealden period, on the leaves and fruit of 

 which they may be presumed to have in great 

 part subsisted. In these reptiles the large 

 flattened and serrated teeth were arranged in each jaw in a single row, but in cer- 

 tain smaller forms known as trachodons, 

 which occur in the higher Cretaceous 

 rocks of both Europe and North America, 

 there were several rows of teeth in use 

 at the same time, the edges of these 

 teeth being so flattened and fitted to- 

 gether that a pavement -like structure 



PAVEMENT-LIKE TEETH OF THE TRACHODON. — 



After Marsh. 



TERMINAL TOE-BONE OF AN ARMOURED 



dinosaur.— After Marsh. 



