Echidna is found in

$A$: The duck-billed platypus and the spiny ant-eater,both are egg-laying animals yet are grouped under mammals.
$R$: Both of them have seven cervical vertebrae and mammary glands without teats.

Jaw suspension characteristic of mammals is

Without teats,mammary glands are found in

The unique mammalian characteristics are:

The absence of a thumb is a characteristic feature of ........