undercoat

A fur seal naturally has fur, the hair seal has no undercoat whatever.

Undercote, or undercoat; fester under the skin (coat is "cutis," skin).

It should be profuse, thick and harsh to the touch, with a dense undercoat.

Get some sort of stuff, made of sheep's clothing, I don't know what you call 'n, for to make an undercoat for the old oman.

She was medium-sized, and her dark undercoat was dappled with bluish spots, or ticks.