102 ELEMENTS OF .COMPARATIVE ZOOLOGY. 



forms in that the red corpuscles are circular in outline and 

 are not nucleated. 



The monotremes form the only exceptions to the state- 

 ment that the mammals bring forth living young. These 

 lay eggs, one species having the eggs about the size of a 

 pigeon; but the young which are hatched from these eggs 

 are nourished by milk secreted by the mother, as is the case 

 with all other mammals. 



The Mammalia are divisible into three large groups or 

 subclasses: Monotremata, Marsupialia, and Placentalia. 



SUBCLASS I. MONOTREMATA. 



This subclass contains three or four species of animals 

 which are found only in Australia and its immediate 

 neighborhood. They present resemblances to the birds, or 



FIG. 38. Duckbill (Ornithorhynchus paradoxus) . From Ltitken. 

 better, to the reptiles in the following points, in all of 

 which they differ from the other mammals: They lay 

 eggs; they have well-developed coracoid bones; and repro- 

 ductive and excretory organs empty into the posterior por- 

 tion (cloaca) of the intestine, and thence pass by a common 

 opening to the exterior. 



