160 MANUAL OF ANIMAL BIOLOGY 



found in the structure of the limbs. These may vary from two 

 pairs of nearly typical pentadactyl limbs, as in Man, to the condi- 

 tion found in the Whale, where the fore limbs are paddle-shaped 

 structures, although maintaining the fundamental pentadactyl 

 arrangement, and the hind limbs are entirely lacking. Or again a 

 reduction of digits may occur, as in the Horse, where only the 

 third digit of each limb is functional. In fact a very complete 

 series of limbs of the various Mammals can be arranged to show 

 the adaptive radiation from a basic, or generalized, type to 

 the highly specialized types in conformity to the chosen environ- 

 mental conditions. (W. fs. 201-207.) 



Again, the mammalian exoskeletal structures show great vari- 

 ation and are used in classifying this group. Thus we have 

 Mammals with claws, or Unguiculata (e.g., Bat, Dog, Bear, 

 Babbit, Squirrel, etc.) ; Mammals with hoofs, or Ungulata (e.g., 

 Horse, Cow, Pig, Elephant, etc.) ; Mammals with nails (e.g., 

 Monkey, Ape, Man, etc.). Mammalian teeth are also a valuable 

 aid in classifying the group. 



In all Mammals, except a few of the most primitive species, 

 the fertilized egg is retained in the body of the mother for early 

 development. It is interesting to note that, in the primitive mam- 

 malian types, large-yolked eggs are laid which are very similar to 

 those of Beptiles and Birds. 



The Mammalia are commonly divided into three subclasses, 

 namely, the Prototheria, the Metatheria, and the Eutheria. 

 The Prototheria are egg-laying Mammals. There are only a few 

 species known (e.g., Ornithorhynchus (Duck-bill) and Echidna 

 (Spiny Ant-eater)) and these are very closely restricted in their 

 distribution. The Metatheria are the pouched Mammals, or 

 Marsupials. The young are born in a very immature condition, 

 and are carried by the mother, for a time, in a special pouch, or 

 marsupium, present on the ventral surface of the female. This is 

 also a very small subclass with the Kangaroo and, in this country, 

 the Opossum as typical examples. (W. fs. 87, 88.) 



The Eutheria, in which the young, owing to the presence of the 

 placenta in the female, are retained for a longer period of uterine 

 growth and are born in a comparatively highly developed condition, 

 include all the remaining mammalian types. These Placentals 

 comprise the familiar species of domesticated animals as well as the 

 Primates. The interest and importance which, therefore, attaches 



