CENOZOIC ERA: AGE OF MAMMALS 631 



apparently occupied a very subordinate position, but which before 

 its close became widespread. Because of the part played by this 

 type of vegetation in the evolution of the most important branch 

 of mammals (ungulates, hoofed mammals) it will be well to discuss 

 the evidence upon w T hich its presence is based. " If we observe the 

 conditions of the preservation of plant remains along existing ponds, 

 river borders, or swamps, we see at once that they are as favorable 

 for the preservation of deciduous leaves as they are unfavorable for 

 the preservation of grasses. Grasses are firmly attached to their 

 roots and are not swept away either by water or wind. Leaf deposits, 

 therefore, abound everywhere and give us sure indications of the 

 forest flora, while we know but little -of the field and meadow flora, 

 which is of great importance in connection with the evolution of the 

 grazing herbivorous ungulates especially. In fact, the evidence as 

 to grasses is very limited throughout the entire Age of Mammals. The 

 number of kinds of grasses (Graminae) found in the whole Cenozoic of 

 Europe is comparatively small, and it is difficult to draw conclusions 

 from fossil plant remains alone as to their relative or absolute im- 

 portance. At what period grasses began to assume anything like 

 their present dominance it is impossible to determine. The absence 

 of native grasses in Australia is indirect evidence of their late geological 

 development." (Osborn.) The indirect evidence of the history of 

 grasses, derived from the adaptation of the teeth of the hoofed mam- 

 mals, " disposes us to adopt the opinion that grasses attained wide 

 distribution in both hemispheres only toward the close of the Eocene. 

 Their evolution on favorable forestless regions was certainly a pro- 

 longed one, beginning in Mesozoic times." (Osborn.) Proof that 

 grasses were widespread in the Miocene is based upon the structure 

 of the mammals ; omnivorous forms were becoming grass eaters ; 

 the method of chewing was changing from a vertical, biting move- 

 ment to a horizontal, grinding one; and the teeth were becoming 

 more durable and better suited for grinding hard food. This then 

 was apparently the time at which the grassy plains began. 



Daemonhelix. — Certain fossils occurring abundantly in the Mio- 

 cene deposits in a restricted area of western Nebraska have given rise 

 to some speculation. They consist of spirals (Fig. 560) of harder 

 rock, held together by fibrous calcareous material which sometimes 

 shows a vegetable structure, and because of their shape and size 

 they have received the name Daemonhelix (Devil's Corkscrew). 

 Some of them are ten feet or more in height and a foot in diameter; 



