- External pinnae are absent.
- In the young stage teeth are present and in the adults horny plates are formed.
- Mammary glands are present and tears are absent.
- Body is covered by hairs.
- Pelvic girdle shows epipubic bones.
- Vertebrae will not show epiphyses.
- Ribs are single headed. (Tuberculum is absent)
- Corpus callosum is absent in brain.
- Cochlea is simple and is not coiled.
- In the males the testis is abdominal.
- Females are egg laying. Hence prototherians are called egg laying Mammals.
This subclass includes only one order that is Monotremata.
Ex: 1. Echidna (Spiny ant eater), 2. Ornithorhyncus (Duck billed platypus)