Mesozoic Reptilian Age. 87 



somewhat resembles a squirrel in size and appearance. The Kangaroo 

 is the largest of the order and is sometimes six feet high. Some of the 

 order are carnivorous, some herbiverous (as the Kangaroo), and others 

 are omniverous. 



The Marsupial brain is devoid of a corpus callosum and the hemi- 

 spheres of the cerebrum are relatively very small. As in Birds and 

 Reptiles, the venous blood is returned to the heart by two principal 

 veins. As in the Batrachians, the pieces of the skull are not united by 

 permanent sutures. The female in some genera has a double uterus, or 

 rather two small uteri, each of which is simply an enlargement of the 

 oviduct and communicates separately with the vagina. See fig. 80. 

 Correspondingly, the male organ, in all except the Kangaroo and Pro- 

 toros, is bifurcated or two pronged, and it has a backward direction, 

 the scrotum being in front of it. The females of all the genera have 

 mammary glands with nipples, and a more or less complete pouch 

 formed by a fold of skin, which encloses the nipples. There is also a 

 bone on each sidLe projecting forward from the pelvis, which serves to 

 protect the intestines from pressure when the young are in the pouch. 

 These are the marsupial bones and are found in the male as well as 

 the female, although the male has no pouch. The young are born in a 

 very immature condition, and fasten themselves upon the teats in the 

 pouch, where they remain fixed till able to care for themselves. The 

 young of the Opossum are born after a period of 26 days uterine ges- 

 tation. They are sometimes 16 in number, blind and nearly shapeless 

 and weigh only a grain each.* They stick to the nipple for about 50 

 days, when their eyes are completed, and they are as large as a mouse 

 and in the condition of new born mammalian infants. They continue 

 to inhabit the pouch till as large as rats. The young Kangaroo when 

 discharged from the uterus is only about an inch long. 



FIG. 72. New born Kangaroo (and teats of mother], 

 About equal to a human embryo of seven weeks. Compare 

 with Fig. 32. 



The Marsupials are called ovoviviparous and 

 non-placental, because they discharge the live 

 foetus from the egg, and do not attach it to the 

 uterus by means of the placenta, as the higher 

 mammals do. 



The Monotremata are so called because, like the birds, they possess 

 but a single orifice for the discharge of excrementitious and reproduc- 

 tive matters. There are two genera and they are found only in Aus- 

 tralia, viz. , the Echidna, which has a long slender snout and feeds on 

 ants, and the Ornithorhyncus, or Platypus. 



*Cuvier, page 90. 



