The family Callitrichidae encompasses the marmosets and tamarins, the smallest of the anthropoid primates and one of the most species-rich of platyrrhine families. Seven new species of Amazonian marmosets (Callithrix, Callitrichidae) have been discovered in recent years, as well as the exceptional dwarf marmoset Callibella humilis. Most of these species were described on the basis of their pelage and presumed separation by major rivers. I performed analyses of craniometric variables by taxa and by river basins, in order to determine if there are significant cranial distinctions between taxa separated by rivers. I analyzed quantitative cranial and mandibular characters of Callibella humilis to determine if it could be distinguished from other callitrichids. I found that Callibella is clearly distinct from all other genera of marmosets and tamarins, in particular in the morphology of the lower jaw. I also analyzed representative species of Amazonian Callithrix and found support for the theory of separation by river-barriers. In my analyses the Amazonian marmosets were divided into three separate species groups, with the easternmost species (Callithrix argentata and C. leucippe) strongly distinct and separated from the others by the broad Rio Tapajos. Two additional species, C. chrysoleuca and C. saterei, formed a discrete group in the central Amazon, and the westernmost species - C. melanura, C. nigriceps and the Rondonia marmoset - formed a third distinct group. These results from cranial morphology align with recent genetic studies indicating that the Amazonian marmosets are strongly divided by the Rio Tapajos, and offer additional support to the theory of river-barriers. Although these species are typically considered to be of low conservation priority, many of them are found in areas experiencing accelerated deforestation. An initial analysis of protected-area coverage for the Amazonian marmosets demonstrates that while some species may be found in a number of protected areas, others are virtually uncovered, and the lack of comprehensive information on their distributions may preclude an effective conservation strategy. The dwarf marmoset Callibella is known from an exceptionally restricted range, with almost no protected areas, and this unique species should be a conservation priority.