454 EVOLUTIONARY MORPHOLOGY 



halves of the neopallium were not connected by that special 

 transverse commissure : the corpus callosum. 



The Monotremes must have diverged from the main stem 

 at this point, and they are represented to-day by Ornitho- 

 rhynchus and Echidna, to which the description just given fits 

 well. They are inhabitants of the Australasian region. 



The remaining mammals were the ancestors of the Mar- 

 supials and of the Placentals. These two groups are fairly 

 closely allied, and have the following characters in common : 

 the mammary glands have teats, they are viviparous, a placenta 

 of some kind is present, the ear has an external pinna, the bones 

 mostly have epiphyses ; the coracoids,interclavicle, and cloaca 

 have been lost. 



It is clear from the reduction of the milk- dentition and of 

 the allantoic placenta (which is only preserved in Perameles) 

 in Marsupials, that they are derived from a stock with two 

 sets of teeth and with a well-formed allantoic placenta. On 

 the other hand, some primitive Placentals show evidence of 

 descent from forms with marsupioid characters, such as 

 alleged traces of a marsupial pouch, of coracoids, and other 

 features. The conclusion to be drawn is that Marsupials and 

 Placentals had a common ancestor perhaps in the Jurassic. 

 Now, in the Jurassic the fossil Trituberculata are found, and 

 they are regarded as related to the Marsupials by some, and 

 to the Insectivores (Placentals) by others. The number of 

 teeth was large. The Trituberculata which derive their name 

 from the pattern of the cusps on the molars were probably 

 related to the common ancestors of Marsupials and Placentals. 

 It is a significant fact that the arrangement of the cusps on the 

 molar teeth in several primitive groups of mammals is of this 

 type, regardless of the diet for which the teeth of the higher 

 members of these groups are modified. Molars with separate 

 cusps like this are called bunodont. 



In the Eocene period, the Marsupials had a wide distribu- 

 tion over the earth, but at the present day they are restricted 

 to the Australian and southern and central American regions. 

 These regions are characterised by their isolation and the 

 comparative absence of mammals of the Placental type. If it 



