The =Vertebrates= form the most characteristic features of the Cretaceous fauna. A revolution has occurred among the Fishes. Sharks of modern type are numerous. Crossopterygians and Ganoidei are rare. There has been an immense expansion of _Teleosts_ or Bony Fishes, which now become the dominant fishes. Most of the Cretaceous Teleosts belong to modern families and even genera. The _Reptiles_, in this period, continue to be the dominant types of the land, air, and sea, and it is difficult to decide whether the Cretaceous or the Jurassic is to be regarded as the culminating period of Reptilian history. The flying Reptiles (Pterosauria) of this period are remarkable for their great size, far exceeding, in this respect, those of the Jurassic time. The land Reptiles (Dinosauria) are in greater numbers than in the preceding period. The sea Reptiles are less numerous than in the Jurassic, but are of greatly increased size. Cretaceous _Birds_ are much more numerous and advanced than the Jurassic ones. The _Mammals_ of the Cretaceous are much more abundant and varied than those of Jurassic times, but they are nearly all of very small size, and continue to play a very modest rôle. The lower Cretaceous mammals differ but little from those of the Jurassic, except for the larger number of genera. The mammals of the latest Cretaceous time are much more numerous and diversified than those of the early Cretaceous. They also show affinities with the mammals of the next succeeding or Tertiary period. The birds and mammals, though they have varied with the passing time and changing surroundings like all other living creatures, are very different from those around us now. They have thrown off some of their reptilian characters, yet they are still distinctly and pronouncedly reptilian. The Triassic, Jurassic and Cretaceous periods together constitute the Mesozoic Era, or =Age of Reptiles=; for never before and never in succeeding ages were reptiles so huge, and varied, and masters in all the realms of nature. =Tertiary Period.= In reaching Tertiary time we enter upon the threshold of our modern world. The reptiles have dwindled to a few low forms, such as alligators and crocodiles, to the small lizards, to turtles and tortoises, and to those low-grade reptiles, the snakes, which now for the first time are the most numerous of the class. In these ages, as now, the Teleost fishes vastly predominate, and the Ganoid and Crossopterygian fishes are nearly extinct. In Tertiary times, as in the present, all the reptilian birds had disappeared, and only typical birds remain. In other words, the bird-class had now separated from the reptilian class, and the connecting links became extinct.