Mr Jemmerling is an egg collector in Great Northern?. He is the owner of the motor yacht Pterodactyl which he uses as a headquarters for his bird and egg collecting expeditions.
He collects eggs, and shows the horrified Dick the dried skin of a large bird, a Great Northern Diver, one of three he has collected on this trip. It is like a feathered balloon from which the air had escaped. There could not have been anything deader. He boasts about the Jemmerling collection, the biggest private collectiion in England, probably in the world. Dick realises Mr Jemmerling is a George Owdon, only grown-up and worse (GN9).
Mr Jemmerling wants to shoot the nesting birds as well as get photos, quoting the bird collectors' maxim What's hit's history; whats's missed's mystery (originally suggested to Arthur Ransome by Myles North). Captain Flint describes Mr Jemmerling as He's not mad but bad. Rotten bad .... He thinks Dick really has got hold of something and he wants to take the credit for it (GN11).
Mr Jemmerling is red-haired, like Ronald, one of the Hullabaloos.