Difference between Prototheria, Metatheria and Eutheria
The mammals are defined are hairy quardrupeds. Their name refers to the milk secreting mammary glands in the females for sucking the young for sometimes after birth. The mammals have evolved from the mammals like reptiles like Dimetrodon in Triassic some 200 million years ago in the Jurassic period, before the first birds. There are 5000 species of living mammals known. 
Prototheria vs Metatheria vs Eutheriai. Prototheria (Egg laying mammals)1. Ear without pinna.2. No nipple 3.No corpus callosum4. Testes abdominal, no scrotum5. Digestive and urinogenital tracts open out through cloaca and cloacal aperture.6. No marsupial pouch7. Oviparous8. No placentaExample of Prototheria: Duck billed platypus (Ornithorynchus anatinus), Anteater (Tachyglossus)
ii. Metatheria (Pouched mammals or marsupials)1. Ear with pinna2. Nipples abdominal3. No corpous callosum4. Scrotum in font of penis5. Digestive and urinogenital tracts open out by separate apertures that are controlled by a common sphincter muscle.6. Marsupial pouch often present7. Viviparous8. Placenta small, intrauterine development brief, young extremely small and helpless, brought up in marsupial pouch, if present.Example of Metatheria: Kangaroo (Macropus), Opossum (Didelphis)
iii. Eutheria (Placental mammals)1. Ear usually with pinna.2. Nipples abdominal or throracic.3. Corpus callosum present.4. Scrotum behind penis.5. Digestive and urinogenital tracts open out by separate apertures, each with its own sphincter muscle.6. No marsupial pounch7. No marsupial pouch8. Placenta large, intrauterine development prolonged, young advanced and fully formed.Example of Eutheria: Rat(Rattus), Horse (Equus)
Prototheria vs Metatheria vs Eutheriai. Prototheria (Egg laying mammals)1. Ear without pinna.2. No nipple 3.No corpus callosum4. Testes abdominal, no scrotum5. Digestive and urinogenital tracts open out through cloaca and cloacal aperture.6. No marsupial pouch7. Oviparous8. No placentaExample of Prototheria: Duck billed platypus (Ornithorynchus anatinus), Anteater (Tachyglossus)
ii. Metatheria (Pouched mammals or marsupials)1. Ear with pinna2. Nipples abdominal3. No corpous callosum4. Scrotum in font of penis5. Digestive and urinogenital tracts open out by separate apertures that are controlled by a common sphincter muscle.6. Marsupial pouch often present7. Viviparous8. Placenta small, intrauterine development brief, young extremely small and helpless, brought up in marsupial pouch, if present.Example of Metatheria: Kangaroo (Macropus), Opossum (Didelphis)
iii. Eutheria (Placental mammals)1. Ear usually with pinna.2. Nipples abdominal or throracic.3. Corpus callosum present.4. Scrotum behind penis.5. Digestive and urinogenital tracts open out by separate apertures, each with its own sphincter muscle.6. No marsupial pounch7. No marsupial pouch8. Placenta large, intrauterine development prolonged, young advanced and fully formed.Example of Eutheria: Rat(Rattus), Horse (Equus)
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