THE CRETACEOUS PERIOD. 185 



are now known from the Cretaceous; but less than half a dozen of 

 them belong to the Hesperornis type. 



Compared with the Jurassic Archceopteryx, both the Hesperornis 

 and Ichthyornis show progress in the abbreviation of the long bilater- 

 ally feathered tail, and in the loss of the distinct fingers and claws; 

 but, on the other hand, the fish-like vertebrae of Ichthyornis, and the 

 groove-set teeth of the Hesperornis, are features almost as primitive 

 and reptilian as any possessed by the Jurassic bird. This illustrates, 

 as noted by Marsh, that certain parts of an animal may linger in a 

 primitive condition, while other parts make notable advances. The 

 wide divergence of the two Cretaceous types from one another, and 

 the divergence of both from the Jurassic form, seem to imply that 

 birds had their origin at a much ear her date. What was happening 

 in all this time among the true land birds is almost wholly unknown. 



The seaward movement. — From the foregoing, it will be seen that 

 a notable feature of the period was the marked movement of land forms 

 to the sea. Besides the ichthyosaurs and plesiosaurs, whose ancestors 

 were land forms which went clown to sea when the Jura-Trias sea 

 extension reduced the land-area, and broadened the shallow seas, 

 there were now added, in this greater period of sea extension and land 

 restriction, the dolichosaurs and pythonomorphs descended from some 

 land form of the scaled reptiles, the sea turtles from the terrestrial 

 chelonians, a marine rhynchocephalian from some land form, and aquatic 

 birds, one form of which was specialized for sea life as perhaps no bird 

 was before or has been since, besides the further marine adaptation 

 of the crocodilians and the pterosaurs, one type of which was also 

 extremely specialized for aquatic life. All this is doubtless but a natu- 

 ral outcome of the prolonged and extensive transgression of the sea 

 upon the face of the continents. 



The marine fishes. — A very important change took place in the 

 fish fauna, in the transfer of dominance from the ganoids and other 

 forms of ancient fish to the teleosts, the present prevailing kind. This 

 change set in during the Comanchean, much as did the change in the 

 plants, and was complete by the middle of the Later Cretaceous, thus 

 running singularly apace with the evolution of the angiosperms. It 

 is not easy to see any genetic relationship between these changes, for 

 the teleosts do not seem to be in any notable way dependent on angio- 

 spermous vegetation. Though modern in type, the special forms were 



