Taken as a whole, the head differs from other types of animals in a blending of characters which at the present day are found among Birds and Reptiles, with some structures which occur in extinct groups of animals with similar affinities, and perhaps a slight indication of features common to the lowest mammals. It is chiefly upon the head that the diverse views of earlier writers have been based. Cuvier was impressed with the reptilian aspect of the teeth; but in later times discoveries were made of Birds with teeth--Archæopteryx, Ichthyornis, Hesperornis. The teeth are quite reptilian, being not unlike miniature teeth of Mosasaurus. If those birds had been found prior to the discovery of Pterodactyles, the teeth might have been regarded as a link with the more ancient birds, rather than a crucial difference between birds and reptiles. All the specimens show a lateral temporal hole in the bones behind the eye, and this is found in no bird or mammal, and is typical of such reptiles as Hatteria. The quadrate bone may not be so decisive as Cuvier thought it to be, for its form is not unlike the quadrate of a bird, and different, so far as I have seen, from that of living reptiles. This region of the head is reptilian, and if it occurred in a bird the character would be as astonishing as was the discovery of teeth in extinct birds. These characters of the head are also found in fossil animals named Dinosaurs, in association with many resemblances to birds in their bones. The palate might conceivably be derived from that of Hatteria by enlarging the small opening in the middle line in that reptile till it extended forward between the vomera; but it is more easily compared with a bird, which the animal resembles in its beak, and in the position of the nares. Excepting certain Lizards, all true existing Reptiles have the nostrils far forward and bordered by two premaxillary bones instead of one intermaxillary, as in Birds and Ornithosaurs. If nothing were known of the animal but its head bones, it would be placed between Reptiles and Birds. CHAPTER IX THE BACKBONE, OR VERTEBRAL COLUMN