The tail looks something like that of a beaver. The nostrils also open through the snout, on its upper half, and are close together. The idea that the platypus has a horny bill like that of a bird arose from examination of dried skins only. The olfactory organs of the platypus are not as well developed as in echidnas. There is no ear pinna.
Adults lack functional teeth and instead have hornlike plates for most of the length of each jaw.
Near the front of the mouth these plates are sharp ridges, but toward the back they are almost flat and function as crushing surfaces. They grow continuously and are rapidly worn by gritty food. The flattened tongue works against the palate and aids in mastication. The bones of the upper and lower jaws expand distally and support the bill. The brain of the platypus is relatively small, with smooth cerebral hemispheres.