THE EOCENE PERIOD. 253 



through the several orders that are now found in the latitude of the 

 middle and southern States. 



The land animals. — As already indicated, the Florissant beds are 

 phenomenally rich in fossil insects, and fishes also were abundant. Both 

 classes had a modern aspect of the middle temperate phase, but all the 

 species of insects, of which over 700 have been described by Scudder, 1 

 are extinct. This seems to indicate that although the types had all 

 become modern, the species continued to evolve with relative rapidity. 

 In this respect the insects stand between the more slowly evolving marine 

 invertebrates and terrestrial plants on the one hand, and the more 

 rapidly evolving mammals on the other. The rapid development of the 

 mammals perhaps finds part of its explanation in their progressive 

 adaptation to the angiospermous vegetation. The mammals continued 

 their rapid evolution without interruption, and perhaps even with some 

 acceleration, assisted by the moderate extension of the land and good 

 migratory connections with Europe. The Carnivora proper came into 

 clear definition, and were represented in the White River beds by ances- 

 tral dogs (Daphccnus, Cynodictis, Cynodesmus), cats (Dinictis, Hoplo- 

 phoneus, Eusmilus), coons (Phlaocyon), and weasels (Bunalurus), while 

 some creodonts remained. The rodents were represented by squirrels 

 (Sciurus), beavers (Steneofiber) , pocket-gophers (fiymnovtychus), rab- 

 bits (Palccolagus), and mice (Eumys). Among the perissodactyls, the 

 rapidly developing horse family presented the forms Mesohippus and 

 Anchippus. There were also lophiodonts (Colodon), tapirs (Protapirus) , 

 rhinoceroses (Leptaceratheriam and Aceratherium) , and the related Hyra- 

 codon and Metamynodon, as well as gigantic titanotheres. The artio- 

 dactyls took on the extinct forms of Anthracotherium , Hyopotamus, 

 Elotherium, and of oreodons, as well as ancestral peccaries (Perchccrus, 

 Thinohyus), camels (Poebrotherium, Protomeryx) , ruminants (Lepto- 

 meryx, Hypertragidus, Hypisodas), and the singularly specialized horned 

 and tusked Protoceras, making the artioclactyls a very important group. 

 There were also insectivores (Ictops, Mesodectes), and marsupials referred 

 doubtfully to the genus Didelphys, the opossum. 2 Many of the fore- 

 going were present also in Europe, where there were also shrews, moles, 



1 The Tertiary Insects of North America, U. S. Geol. Surv. Ter., Vol. XIII, 1890; 

 Mon. XXI and XL, U. S. Geol. Surv., 1893 and 1900. 



2 The classification of W. D. Matthew is here followed. Bull Am. Mus. Nat. 

 Hist., Vol. XII, 1899, Art. III. pp. 19-75. 



