EVOLUTION OF MAMMALIA. 173 



dentia, remarkable transitional forms have been discovered among 

 the remains of extinct types. These also appear most prominently 

 among the tertiary remains of North America. In the Eocene 

 period here (Wyoming) lived the Tillodontia with the genus Tillu- 

 therium,* characterized by having a broad skull like a bear, two broad 

 incisor teeth like a rodent, and molar teeth like Palceotherivm, and 

 feet having five toes armed with strong claws. It thus united in 

 its skeletal structure peculiarities of Carnivora and Ungulata. The 

 Dinocerata (Dinoceras laticeps mirabile) were powerful Ungulates 

 with five-toed feet with six horns on their heads, without incisors in 

 the prsemaxillary bone, with strong sabre-like canine teeth in the 

 upper jaw and with six molars. 



A third type, that of the Brontotheridce attained elephantine 

 proportions, and was provided with transversely placed horns in front 

 of the eyes. In addition to the foregoing there are a number of 

 other groups of Mammals now completely extinct, the remains of 

 which extend back into far earlier strata. Amongst them are the 

 South American Megatheridce (Mylodon, Megatherium], which belong 

 to the order Edentata, and the Toxodonlia, whose skull and dentition 

 show relations to the Ungulates, Rodents, and Edentates. Many 

 other types, however, especially of the Ungulates, which during the 

 tertiary period inhabited both hemispheres, are now extinct in 

 America, but still exist in the East. Elephants, Mastodonta, 

 Rhinoceridse, and Equidse existed in America in the diluvial but 

 not in recent periods. Of the Perissodactyles the group of Tapirs 

 alone is preserved in America. This group has also been preserved 

 in the Eastern hemisphere in the East Indian species. 



In the palsearctic region also are found the remains of extinct 

 intermediate groups of Mammals which existed during the tertiary 

 period. In the Phosphorites of Quercyt in the south of France are 

 found the remains of the skulls of Prosimise (Adajns), the dentition 

 of which is intermediate between the ancient Ungulates and the 

 Lernurida; (Pachylemuridce], so that the question may be raised 

 whether the Prosmriae had not a common ancestry with several 



* Compare 0. C. Marsh, "Principal Characters of the Tillodontia." Amei: 

 Journal of Science and Art, Vol. xi., 1876. 



0. C. Marsh, "Principal Characters of the Dinocerata." Amcr. Journal oj 

 Science and Art, Vol. xi.. 187(5. 



0. C. Marsh. " Principal Characters of the Brontotheridse." Amer. Journal 

 of Science and Art, Vol. xi., 1876. 



f Compare H. Filhol, ' Recherches sur les Phosphorites du Quorcy, fjtiulc 

 des fossils qu'on y rencontre et specialement dea Mammifcres." Ann. Science* 

 i'S, Vol. vii., IS 76. 



