PEDIGREES AND RELATIONSHIPS 29 



their teeth and diminished the length of the tail, it can 

 scarcely be said that the group exhibits much signs of progres- 

 sive evolution above its Liassic prototype. Indeed, there is no 

 reason that it should, seeing that the species in question ap- 

 pears to be thoroughly adapted in every particular for a life 

 in the air and predatory habits. In some of the gigantic 

 Cretaceous forms alluded to in the sequel, specialisation, in 

 correlation with the enormous dimensions of the wings, shows 

 itself in the welding of the scapula to the dorsal vertebra. 

 Another form of specialisation is shown by the development 

 of a ring of bones in the eye. 



The dinosaurs (Dinosauria), like the ichthyosaurs and plesio- 

 saurs, commenced in the Trias and continued to flourish till the 

 close of the Cretaceous period, when some of their members, as 

 is more full)' noticed in the sequel, underwent some most strange 

 and bizarre developments. It is specially important to notice 

 that all the known Triassic forms belong to the typical carni- 

 vorous group (Dinosauria proper), commonly known as Thero- 

 poda ; the more specialised herbivorous types being a later 

 development. It is largely on account of this fact that all the 

 groups are included in a single order in the present work ; 

 although many naturalists prefer to consider the herbivorous 

 forms as constituting an order by themselves. Another very 

 remarkable circumstance is that some of the Triassic represen- 

 tatives of the order had already assumed the bipedal mode of 

 progression. This early assumption of the erect posture makes 

 it somewhat difficult to accept a theory suggested by Professor 

 H. F. Osborn, of New York, to the effect that birds were prob- 

 ably an offshoot from the dinosaurian stock before the posture 

 in question had been attained by any of the members of the 

 latter. The huge herbivorous dinosaurs of the group Sauropoda 

 are known to have been in existence in the early part of 

 the Jurassic period (exclusive of the Lias) ; while the armoured, 

 or stegosaurian, section of the second great herbivorous 

 group (Ornithopoda) date from the Lower Lias ; these Liassic 

 forms being much less fully armoured than were the Jurassic 

 and Cretaceous successors. The highly specialised iguanodonts 

 date apparently from the Upper Jurassic and Wealden. 



To attempt to make a census of the known forms of extinct 

 reptiles would be, in the first place, a matter of extreme diffi- 



