Ornithorhyncus (Duck billed platypus) Ornithorhyncus (Duck billed platypus)
Mammalia is divided into two subclasses. 1) Prototheria 2) Theria
  1. External pinnae are absent.
  2. In the young stage teeth are present and in the adults horny plates are formed.
  3. Mammary glands are present and tears are absent.
  4. Body is covered by hairs.
  5. Pelvic girdle shows epipubic bones.
  6. Vertebrae will not show epiphyses.
  7. Ribs are single headed. (Tuberculum is absent)
  8. Corpus callosum is absent in brain.
  9. Cochlea is simple and is not coiled.
  10. In the males the testis is abdominal.
  11. Females are egg laying. Hence prototherians are called egg laying Mammals.

This subclass includes only one order that is Monotremata.
Order Monotremata:
Thesemammals show prototherian char­acters. They can found in Australia, Tasmania and New-guinea.
Prototheria echidna thumb17

Ex: 1. Echidna (Spiny ant eater), 2. Ornithorhyncus (Duck billed platypus)
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