The extant members of the family Callitrichidae are among the smallest primates and represent the smallest true monkeys (simian primates). Adult body mass ranges between 3.9-21.9 oz (110-620 g), head and body length between 5.5-11.4 in (14-29 cm), and tail length between 7.9-15.8 in (20-40 cm). The non-prehensile tail is always longer than the head and body combined. The head is relatively rounded and the snout only slightly pronounced. Except for lion tamarins and Goeldi's monkeys, females are usually larger than males (in contrast to the general trend in primates where males are larger than females), although to a varying degree. The coloration of the dense, silky fur varies considerably between species. Some, like the golden lion tamarins (Leontopithecus
rosalia), are very conspicuously colored, others, like Graell's black-mantled tamarins (Saguinus graellsi), are rather dull, and still others show contrasting coloration in different regions of the body. The fur may form a kind of a mantle on the shoulders or a mane in some species. Several marmosets possess ear tufts, while in a few tamarins either the beard or the hair on the crown is quite elongated. Males and females are colored alike. Claw-like nails grow on all fingers and toes except the big toe that has a flat nail. Histological studies revealed that these claws are laterally compressed nails, different from the true claws of other mammals. The thumb cannot be opposed to the other fingers, but the big toe is opposable, as in all other primates. The arms are shorter then the legs, but relative length varies between species, depending on the principal mode of locomotion. The third molar is lacking both in the upper and lower jaw; hence the number of teeth is 32. Goeldi's monkey represents the only exception: it possesses small, third molars, and thus 36 teeth. In tamarins, lion tamarins, and Goeldi's monkey the lower canines are higher then the lower incisors (long-tusked callitrichids), while in eastern Brazilian marmosets and pygmy marmosets they attain the same height (short-tusked callitrichids); Amazonian marmosets are intermediate in this character.
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