456 H. F. Osborn — Mammalia in North America. 



the Cernaysian of France. The Multituberculates of the 

 Laramie include the Plagiaulacidse, represented by Ptilodus, 

 the form with two premolars, and Meniscoessus, with two pre- 

 molars and crescentic tubercles. Meniscoessus has a smaller 

 fourth premolar, and is found to lead off to the huge plagiau- 

 lacid Poly mastodon of the Puerco. The only other Multitu- 

 berculates found are those related to Bolodon of the Jurassic 

 and Chirox of the Puerco. The other mammals of the Lara- 

 mie range from the mouse to the opossum in size ; they have 

 superior molars of the simple tritubercnlar type- — the low 

 cusped or bunodont molar predominating in the upper jaw, and 

 the tuberculo-sectorial in the lower. The dental formula is 

 mostly the typical p. 4, m. 3. Yet, judging by the angular 

 region of the jaws, we have here both Placentals and Marsu- 

 pials. Some of the teeth remind us strongly of those in the 

 Puerco ; their determination, however, is very difficult, for 

 the jaws and teeth are almost entirely isolated. From another 

 exposure of the Laramie, Cope has recently found the remark- 

 able type Thlaeodon — remarkable because it is a highly special- 

 ized trituberculate of typical dentition with a jaw which bears 

 resemblance to that of the Multituberculates and of Ornitho- 

 rhynchus. There is no placental angle nor strong marsupial 

 inflection. This raises the supposition that Thlseodon may be 

 one of the persistent trituberculate Monotremes which we are 

 now looking for. 



In the Puerco or basal Eocene, a very marked change occurs, 

 for the American fauna loses some of its cosmopolitan character, 

 the Multituberculates or monotremes die out and the marsu- 

 pials are not found at all ; in fact they do not reappear in 

 North America until the Miocene. 



Ancient and Modem Placental Differentiation. 



The Puerco is essentially an archaic fauna and is to be 

 regarded as the climax of the first period of placental differen- 

 tiation, a culmination of the first attempts of nature to establish 

 insectivorous, carnivorous and herbivorous groups. These at- 

 tempts began in the Cretaceous, and some of the types thus 

 produced died out in the Puerco, some in the Wahsatch and 

 Bridger ; only a few flesh-eaters survived to the Miocene. It 

 is most important to grasp clearly the idea of this functional 

 radiation in all directions of this old Puerco fauna, resulting 

 in forms like the modern insectivores, rodents, bears, dogs and 

 cats, monkeys, sloths, bunodont and selenodont ungulates, and 

 lophodont ungulates. This was an independent radiation of 

 placentals, like the Australian radiation of marsupials. What 



