The Tetrapoda: Manunals 633 



survive. The most famous of these is the well-known and abundant 

 Virginia opossum, Didelphis virginiana, a species which is extremely 

 generalized in its structure. It is usually considered to be almost iden- 

 tical with the type of animal from which the many diverse marsupials 

 developed. 



The Virginia opossum at first sight appears to be a helpless primi- 

 tive sort of animal which by all rights should be unable to compete 

 with more highly developed mammals and with man. Surprisingly 

 enough, not only has it survived, but in recent years has become even 

 more abundant. Perhaps this can be attributed to its generalized struc- 

 ture and nocturnal habits. 



In Australia, the marsupials have developed and diversified so as 

 to fit all possible habitats. This development is traceable to the early 

 separation of Australia from the rest of the world. The marsupials 

 were thus able to evolve without competition from the more aggressive 

 placental mammals. Some of these marsupials include the marsupial 

 cat, Dasyurus; the marsupial wolf, Thylacimis; the marsupial mole, 

 Notoryctes; the phalangers which resemble flying squirrels ; the mar- 

 supial bear, the koala, Phascolarctos ; and the kangaroos and wallabies 

 which correspond in habits to our prairie ungulates. These kangaroos 

 vary all the way from the huge grassland kangaroos to the tiny tree- 

 climbing kangaroos. 



Order Insectivora. — -There is no one specific character which dis- 

 tinguishes the members of this order from those of others, rather it is 

 a series of characteristics. In general, they are small forms with a 

 somewhat prolonged snout. The eyes are small, the brain is simple, 

 the teeth are low and have sharp cusps, and there are usually five toes 

 on each foot. 



The most familiar forms are the moles which burrow through the 

 soil. They are real specialists in underground excavating. The front 

 legs are very powerful and broad for digging, the fur is dense, the 

 eyes are reduced, and the pinna of the ear is absent. The common 

 prairie mole, Scalopus aquaticus, is very abundant throughout the east- 

 ern half of the United States. Their burrows are visible above 

 ground as broken ridges of dirt. These result from the activities of 

 the animals as they search for food near the ground's surface. This 

 food consists chiefly of earthw^orms and insect larvae. 



Some of the members of the family Soricidae, the shrews, are the 

 smallest of the known mammals. In the United States, several genera 



