456 EVOLUTIONARY MORPHOLOGY 



had not been for the latter fact, there is little doubt that the 

 Marsupials would have become extinct, for they cannot com- 

 pete with the Placentals. Instead, in the security of their 

 isolation, they radiated out into a number of types which are 

 especially interesting in that they have evolved parallel with 

 several groups of Placentals, and by becoming adapted to 

 equivalent biological environments have developed a con- 

 vergent resemblance to these Placentals. Nearly all Marsupials 

 have a marsupial pouch in the female and epipubic bones 

 supporting it. 



The opossum (Didelphys) and Cagnolestes are American ; 

 all the remainder are restricted to Australasia, though fossils 

 related to these are also found in South America. Dasyurus 

 is the Marsupial equivalent of the cats, while the dogs are 

 represented by Thylacinus, Perameles (the bandicoot) is an 

 attempt at a rabbit, Petaurus (the phalanger) resembles the 

 flying squirrels, while Notoryctes is a remarkable imitation of 

 the mole. Phascolarctos (the koala) is the " marsupial bear," 

 Phascolomys (the wombat) is the " marsupial rodent," the 

 extinct Thylacoleo was the " marsupial lion," while Macropus 

 (the kangaroo) represents the swift-moving Ungulates. 



The Cretaceous strata of Mongolia have revealed fossils of 

 apparently Placental mammals, of which Deltatheridium is an 

 example, and which can be regarded as intermediate between the 

 Jurassic Trituberculata and the true Placentals of the Eocene. 



The Placentals are characterised by the possession of an 

 allantoic placenta, a corpus callosum joining the two halves of 

 the neopallium, and a typical dental formula of if, c], p|, m|. 

 This number of teeth is, however, often modified and reduced. 



At the beginning of the Eocene period there appeared a 

 group of true Placentals which were primitive in that they were 

 of small size, with tritubercular short-crowned molars, five 

 fingers and toes, and walking on the flat of the hand and foot. 

 Among them can be recognised some with a tendency to 

 modification of the teeth for a carnivorous diet — the Creodonta, 

 — others for a herbivorous diet — the Condylarthra. Others 

 again were generalised Insectivora. Very early, a branch 

 diverged from the Condylarthran stock and gave rise to the 



