ADAPTATIONS TO SPECIAL ENDS 129 



Lightness is also produced by the adoption of the T-iron prin- 

 ciple in the construction of the trunk-vertebrae of these reptiles, 

 which show a number of buttress-like supports of this nature. 

 In consequence of this arrangement the vertebrae of these 

 gigantic herbivorous dinosaurs did not weigh much more than 

 half as much as those of whales of equal bulk ; and it is probable 

 that the entire weight of the largest species was not more than 

 some forty or fifty tons instead of sixty or seventy. 



Honey-combed and buttressed vertebrae of this type are 

 found only in the so-called sauropod dinosaurs, which walked 

 on all fours, and were probably to a great extent aquatic. 

 Indeed, if their habits were not aquatic, it is doubtful if these 

 reptiles could have attained such huge size. The need of 

 lightness in the skeleton appears, however, to have been equally 

 felt in the carnivorous, or theropod, group, most, if not all, 

 the members of which habitually walked in the upright pos- 

 ture. In the case of species like the large megalosaurs, which 

 probably stood fifteen or twenty feet high, lightness of skeleton 

 was essential on account of the great bodily size ; but in 

 certain much smaller forms, such as Ccelurus and Calamo- 

 spondylus, it appears to have been required in order to admit 

 of great bodily activity and speed, some of these small dinosaurs, 

 as already stated, having perhaps been in the habit of catching 

 and killing the contemporary long-tailed birds. Be this as it 

 may, in Megalosaurus and its relatives both the limb-bones and 

 the bodies of the vertebrae were hollow internally (and probably 

 filled with marrow during life) ; while in Ccelurus and Calamo- 

 spondylus the entire vertebrae consisted of little more than a thin 

 shell, the whole interior being hollow. As the vertebrae of 

 these theropods differ markedly in structure and the manner in 

 which they are hollowed out from those of the sauropods, it is 

 clear that the lightening process has taken place independently 

 in the two groups. This, however, is by no means all, for the 

 iguanodonts, or bipedal herbivorous dinosaurs, in which the 

 vertebrae are solid, also have hollow limb-bones, and these 

 would appear to have acquired this structure independently of 

 the carnivorous group. 



One other group of reptiles, namely the pterodactyles (Orni- 

 thosauria), to which lightness of body was a matter of vital im- 

 portance, achieved it in the same manner as birds ; their bones 



9 



