25« 



NA TURE 



[January i i, 1894 



diphyodont stage, with numerous molars, and secondarily sup- 

 pressed the first heterodont series, and established a 

 numerous homodont second series. 4. The Cetacea also 

 branched off from a diphyodont, heterodont stage, and second- 

 arily established a numerous homodont first series, suppressing 

 the second series. 



Breaks and Links in the Mesozoic Fauna. 



Bv our hypothesis all three sub-classes flourished together 

 during the American Mesozoic ; the Marsupials disappeared, 

 then the Monotremes, and by the end of the basal Eocene the 

 Placentals were in exclusive possession of the northern 

 continent. 



Although we have great reason to congratulate ourselves 

 upon the rapid progress of discovery, there still remain great 

 gaps in Mesozoic time between certain horizons, and in the lineal 

 phyletic series of both the Mesozoic and Cenozoic. For a 

 time standard we may take advantage of the remarkably con- 

 stant evolution of the Plagiaulacidce in the Mesozoic, and of 

 the Equidas in the Cenozoic — as certain invertebrates are 

 made use of in older rocks. The grooves and tubercles of 

 Plagiaulax and the cusps and styles of the horses are added 

 with the precision of clockwork, and supposing that the rate 

 of evolution has been about the same, we can approximately 

 estimate both the periods of deposition and the intervals as 

 follows. 



PLAGIAULACID^. 

 Stonesfield. Purbeck. Laramie. Puerco. Cernaysian. 



Number of Premolars 

 Grooves on Premolars, 

 Molar Tubercles : outer 

 inner ; 



4- 



7-9 



2-1 

 12-15 



11-14 



4 '. 2 6 : 4 6:4 g ; 6 



Estimating the geological intervals by dental evolution and 

 faunal succession, there is first the great gap between the 

 Trias of Microlestes and Dromatherium and the Jurassic of 

 the Stonesfield slate ; there is a relatively shorter interval, but 

 still a considerable one between this and the Purbeck or Atlan- 

 tosaurus beds. Then follows another long and very important 

 interval between the Atlantosaurus beds and the Laramie 

 (Upper Cretaceous). The gap between the Laramie and 

 Puerco was relatively short, as indicated by the comparatively 

 limited evolution both of the Plagiaulacids and Trituberculates. 

 The Puerco was itself a long period in which the Plagiaulacids 

 underwent considerable changes. Then follows an interval 

 which it is most important to fill by future exploration, for 

 between the Puerco and the Wahsatch the diff"erentiation of 

 the even and the oddtoed ungulates must have occurred. The 

 Wahsatch proper does not mark a very extensive evolution of 

 the forms it contains. It passes, after a slight break, into the 

 base of the Bridger (Wind River), and then begins that splendid 

 and almost uninterrupted succession of lake basins, terminat- 

 ing in the pliocene. I append a table, to be compared with 

 that published by Marsh in his admirable address of 1877, and 

 which exhibits the great progress of the last sixteen years. 



THE SUCCESSION OF THE NORTH AMERICAN MAMMALIA. 



Lower. 



EOCENE. 



Upper. 



Middle. 



Lower. 



Oreodon. 



(Middle.) 

 TiTANOTHERIUM. 



(Lower.) 



Uinta. 



BRIDGER. 



Washakie. 



(Upper.) 

 Bridger. 

 (Middle.) 



Wind River. 

 (Lower.) 



WAHSATCH. 



Characteristic Genera. New Types Appearing. 



Eqinis, 5 species. Elephant, E. pritiiigenius. Mastodon. Llamas. 

 Camels, Eschatius. Holoiiieniscus. Elk, Alces. Platygonus. 

 Sloths, Mylodon, Glyptodon. Ursiis. 



Equiis, 3 species. Mastodon, 3 sp. Llamas, Pliaiichenia, Platy- 

 gonus. Sloth, Megalonyx, Felidcu. (?) Hycenid{^. MnstelidcE. 



Protohipptis, Hippaiion. Mastodon. Rhinoceroses, Aphelops, 5 

 species. Canidcr. Fclida. Rodents. Edentates. Camels and 

 Llamas, Procameliis, Protolabis. Oreodons, 3 genera. Deer, 

 Blastovieryx, Cosoryx. 



Protohippiis, Anchithertum. First Mastodons. Oreodons, Cyclo- 

 pidiiis. C halicotherunn. Tylopoda. 



liliohippus. Two-horned Rhinoceros, Dicerailierinin. Hyopota- 

 ni7is. Peccaries. Oreodons. Rodents. Canidcr, Felida;. 



Tylopoda. 



Appearance of tragulines, Elotheres, 

 Hyopotatniis, pigs and peccaries 



Types becoming Extinct. 



(?) liliohippus. Artionyx. 



Mesohippiis. Amynodon. 



Mcsohippns. Titanotherium. 



true dogs, cats, monkejs. Lcp- 

 taiicJienia. Colodon. Chalico- 

 theriiim, Aceratherimn, Prota- 

 piriis, Agriocliocrus. Opossums. 

 Tylopoda, Pochrotheriiim. Creo- 

 donts, HycEnodon. Rodents. In- 

 sectivores. 



Epihippus. Amynodon. Tilanothere, Diplacodon. First Oreo- 

 dons, Protoreodon. First Tylopoda, Lepiot?-agutiis. Tapirs. 

 Hyracodons. Rodents. Creodonts, ]\Iesonyx. 



Pachynolophus. Appearance of Amynodons and horned Titano- 

 theres. Palceosyops. Hyrachyiis. Tripiopus. Achienodnn. 



Pac/iynoiophus. Appearance of Insectivora, Cheiroptera, Hyra- 

 codons. Uintatlieriiem. Palceosyops. Creodonts. 



Dinocerata. 



Hyracotherinin. Palceosyops. 

 codiLS. 



Coryphodon. Phena- 



Hyracotheriui)!. Appearance of Artiodactyls, Perissodactyls : tapirs, 

 horses, titanotheres, lophiodons. First Rodents. First Corypho- 

 dons, Lemurs and Monkeys. Creodonts, 6 families, Palcconictis. 



Interval. 



PUERCO. 



Interval. 

 UPPER CRETACEOUS. LAFAMIE. 



Interval. 



MIDDLE JURASSIC. 



UPPER TRIASSIC. 



Atlantosaurus. 



Interval. 



Chatham Coal Beds. 



(Differeniiation of jnodem clawed and hoofed placentals^ 



Ptilodus, Neofilagiaulax, Polymastodon. Ancient types of Ungu- 

 lates, Carnivores and Insectivores : Amblypoda, Condylarthra, 

 Creodonla. Tasniodonta. Tillodontia. Lemurs. 



{^Differentiation of ancietU clawed and hoofed placentals.) 



Ptilodus. Bolodoniidcc (Multituberculates). Thlceodon, Trituber- 

 culate Placentals and Marsupials. Typical dentition. 



Ctenacodon, Plagiaiilax, Bolodon, Multituberculates (? Mono- 

 tremes). Triconodonts (? Marsupials). Trituberculates (? Pla- 

 centals). Primitive dentition. 



Protodonta, Dromatherium, Microconodon, primitive Triconodonts 

 (? Monotremes). 



NO. 1263, VOL. 49] 



Extinction of Oreodons and 

 hornless rhinoceroses. 



Disappearance of Chalico- 

 therium. Extinction of 

 Creodonts, Hyaenodons. 



Extinction of Elotheres and 

 Hyopotamus 



Extinction of Hyracodons. 



Extinction of Amynodons. 

 Extinction of 1 itanotheres. 



Extinction of Dinocerata, of 

 some Creodonts. 



Extinction of Tillodontia. 



Extinction of Coryphodontia 

 and Condylarthra. 



E.\tinction of Arctocyons. 



Extinction of Multitubercu- 

 lates (? Monotremes). 



Disappearance of Marsupials. 



