TERTIARY ANIMALS. 529 



earlier or Cretaceous times. The origin of whales is not known, al- 

 though they probably came from land mammals by retrograde changes 

 adapted for aquatic life. 



The Atlantic and Gulf border strata are of course all marine, and 

 therefore contain very few land-animals. It is to the fresh-ivater ba- 

 sins of the interior that we must look for a full record of the mam- 



Fig. 907.— Vertebra and Tooth of Zeuglodon cetoides, reduced. 



malian fauna of America in Tertiary times. These basins furnish the 

 fullest and most continuous record of the whole Tertiary which has 

 ever yet been found. The Early Tertiary fauna of America was wholly 

 different in species and in genera, and even largely in families, from 

 that of Europe. This shows that the two continents were then as now 

 widely separated. It will be best to take them in the order of their 

 age, as we can thus best show the evidences, if any, of derivation of the 

 later from the earlier faunae. 



4. San Juan Basin— Puerco Beds — Lowest Eocene.— In these, the 

 very lowest part of the Lower Eocene — so low that they are regarded by 

 some even as partly Laramie — Cope has found a great number of most 

 extraordinary mammals, more generalized than any before known. 

 These earliest true mammals seem to have been very abundant, for, out 

 of 106 species of vertebrates found, 93 were mammals. They represent 

 already all the main divisions of the Mammalian class. Besides marsu- 

 pials continued from the Mesozoic, there were Carnivores, Herbivores, 

 Insectivores, and Lemurine Primates ; but in forms so generalized that 

 they scarcely deserve these names, and may well be called Pro-Car- 

 nivores, Pro-Herbivores, etc., or progenitors of these now widely-dis- 

 tinct orders. The remains of these animals are not so perfect as those 

 on higher horizons. We select for illustration an almost perfect hind- 

 limb of the Periptychus of Cope, one of the most generalized of known 

 animals. It is seen that the foot-structure is perfectly generalized and 

 the tread completely plantigrade. It is an admirable example of the 

 primitive foot. 

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