They are entirely black with long hair that sticks out around their heads. Their head and body length is 8-9 inches, while their tail length is 10-13 inches. they weigh 14-19 ounces. Goeldi’s have claws instead of flattened nails on their fingers, a trait they share with tamarins. Also, Goeldi’s are the only small new world monkeys with 36 teeth; all other marmosets and tamarins have 32. Goeldi's monkeys are a zoological puzzle. They are small and look like tamarins and, like them, have claws rather than nails on the fingers. But Goeldi's teeth and skull shape are more like the bigger New World primates, such as capuchins. It is now thought that Goeldi's belong in a separate group of primates, distantly related to both tamarins and capuchins.