442 EVOLUTIONARY MORPHOLOGY 



(Jurassic), reaching the immense length of 90 feet. Others 

 were carnivorous with formidable teeth, and the hind limbs 

 larger than the fore limbs, so that they were probably bipedal. 

 An example of such a form is Tyrannosaurus, Cretaceous, 

 reaching almost 50 feet in length. The foregoing types of 

 Dinosaurs had a pelvis of normal shape, and are grouped 

 together under the term " Saurischia." The remainder, Pre- 

 dentata (or Ornithischia), have an additional postpubis which 

 stretches back beneath the ischium, and a predentary bone 

 in the lower jaw. The Predentata include the herbivorous 

 bipedal Iguanodon from the Cretaceous, over 30 feet long ; 

 the absurd-looking Stegosaurus with its armour of large bony 

 plates (Jurassic, 20 feet long) ; and the horned Triceratops 

 (Cretaceous, 25 feet long). 



The Pterosaurs were closely allied to the Dinosaurs, and 

 like them had a pair of temporal vacuities and a prelachrymal 

 vacuity on each side. They also preserved gastralia, and while 

 some had teeth others were toothless. The fore limbs were 

 modified for flying, by means of a web of skin stretched from 

 the greatly elongated fourth finger. The Cretaceous Ptera- 

 nodon had an expanse of wings measuring 25 feet. Of all this 

 enormous wealth of reptilian life which dominated the land, 

 water, and air, in the Jurassic and Cretaceous periods, only 

 the Squamata, the Chelonia, the Crocodilia, and the Rhyncho- 

 cephalia have survived, and in very reduced numbers. The 

 rest went extinct before the Eocene. It may be that a reduc- 

 tion of temperature put an end to them, or that the food 

 supply became deficient. Certain it is that their brains were 

 ridiculously small, and they can have been no match for the 

 small and agile mammals in intelligence. 



Two main points for consideration arise out of a study of 

 the reptiles. The first concerns the structure of the 5th 

 metatarsal bone. In the Cotylosaurs it was of a normal shape, 

 as also in the Synapsida. Now, the Synapsida are to be 

 regarded as having been derived from the Cotylosaurs, and one 

 group of them, the Theromorphs, gave rise to the mammals. 

 The line of mammalian descent is therefore characterised by 

 the possession of a normal-shaped straight 5th metatarsal. 



