The type genus of the marmoset family is Callithrix, and the scientific name of the family is therefore 'Callitrichidae'. If we're honest, they're not really any more 'typical' than most other species in the family, but they're as good a place to start my survey of the world's marmosets and tamarins as anywhere else.
The Callithrix monkeys are true marmosets, a group that is distinguished from other members of the family by a heavier reliance on tree gum as a source of food. All callitrichids eat gum, but for true marmosets, it's a vital part of their diet that they eat all year round. Having evolved to do this, they have a number of physical adaptations that suit them for the lifestyle. Unlike their relatives the tamarins, they have no tusks in their lower jaws, meaning that the teeth at the front of their mouths form a relatively straight line. They feed by clamping the teeth of their upper jaw onto tree bark, and then scraping upwards with that flat row of teeth on the lower jaw.
Although they aren't born that way, over time, these teeth are honed to a sharp chisel-like edge. However, for this to work, they also need a remarkably wide gape, and their jaw and its muscles are adapted accordingly. Nor does it end there, because there's the business of digesting the gum.