771 Comparative Morphology of Chordates 



two lines, it may be said that along one line anything of placental 

 nature which may have been initially present was all but suppressed 

 and marsupial reproduction became the dominant feature of the group. 

 Along the other line, any marsupial propensities which may have been 

 present at the start were suppressed and placental reproduction became 

 perfected. It is quite possible that the two lines had entirely separate 

 origins and that such similarities as exist between the reproductive 

 mechanisms of the members of the two lines are instances of parallel 

 but independent evolution. 



At the end of the Cretaceous, the marsupials were widely dis- 

 tributed in both hemispheres. Apparently, however, they were unable 

 to hold their own against the more aggressive placentals and suffered 

 almost complete extinction, surviving only in the Australian region. 

 where a few rodents and bats are the only native terrestrial placental 

 mammals, and in South America and southeastern North America, 

 where a few small marsupials (opossums and Caenolesles) survive, 

 apparently by retreating from competition. 



It is a most noteworthy fact that the marsupials eventually differ- 

 entiated into various types of mammal, each type having distinctive 

 characteristics wherein it closely resembles an analogous placental 

 group. Many of the smaller marsupials externally resemble placental 

 insectivores or rodents. In many cases the resemblance to rodents is 

 increased by the prominence of the incisor teeth. The South American 

 Caenolestes is called the "marsupial shrew." The blind burrowing 

 Notoryctes (Fig. 592) is the "marsupial mole," having all of the 

 specialized features of a placental mole. The banded anteater (Myr- 

 mecobius), about as large as a cat, has the long snout and long, sticky 

 tongue which always go with anteating. There is a long-eared "rabbit 

 bandicoot," and the "flying" phalangers (Fig. 593), of which there are 

 several species ranging in size from that of a mouse to that of a cat, 

 have "flying" (i.e., sailing) membranes similar to those of "flying" 

 squirrels and the colugo (Order Dermoptera). The wombat (Phas- 

 colomys), 2^ to 3 feet long including a 2-inch tail, looks like an over- 

 grown rodent. The arboreal koala (Fig. 542) is the "marsupial bear." 

 Among the polyprotodonts are many marsupial carnivores resembling 

 placental carnivores in appearance, teeth, and habits. The dasyures 

 (Fig. 535) are more or less catlike. The Tasmanian wolf (Fig. 536) has 

 a doglike head on a more catlike body. The Tasmanian "devil" 

 (Sarcophilus), short in the legs and resembling a badger, is said to 

 emit a frightful growl and has a reputation for great ferocity. The larger 

 kangaroos (Macropus) correspond to the ungulates in being large and 

 herbivorous, but there the similarity ends — the methods and mechan- 

 isms of locomotion in cow and kangaroo are about as unlike as possible. 



