Class Mammalia is sub-divided into three sub-classes : Prototheria, Metatheria and Eutheria.
Sub Class 1-Prototheria:
(1) These are primitive mammals, which are the connecting link between reptiles and higher mammals.
(2) They have hairy skin but lack external ears.
(3) Their mammary glands are without teats.
(4) They lay eggs, covered with calcareous shell, e.g. Echidna, Duckbill platypus.
Sub Class 2-Metatheria: They are characterised by the presence of pouch or marsupium on the belly of females in which the young bom in the immature state are kept and nourished on milk produced by mammary glands, e.g. Kangaroo.
Sub Class 3-Eutheria:
(1) They are higher mammals.
(2) The young ones develop inside uterus of females, where they are nourished through a special structure called placenta.
(3) The youngs have a intra-uterine development and
are born in advanced stage, e.g. Pteropus, Lemurs, Cat, Dog, Horse, Elephant.