New World Monkey Morphology

From Primatome.com

Living New World Monkeys are all small to medium size primates, ranging form just under 100g to just over 10kg. Their name is derived from the broad, flat shape of their external nose, which often, but not always, distinguishes them from the Old World anthropoids, which often have narrow, or catarrhine, nostrils. In dental and cranial anatomy, platyrrhines have many primitive features that have been lost in the evolution of Old World catarrhines. For example, they heve three premolars, and the tympanic ring is fused to the side of the auditory bulla but does not extend laterally as a bony tube. 

They also share some unique specializations of their own. The first two lower molar teeth of living platyrrhines usually lack hypoconulids, and on the lateral wall of the skull (the pterion region) the parietal and zygomatic bones join to separate the frontal bone above from the sphenoid below. In addition, the cranial sutures of platyrrhines fuse relatively late, and many species have relatively long narrow skulls.

Platyrrhine limb proportions are relatively conservative, with intermembral indices ranging between 70 and 100. Platyrrhines lack the extremely low intermembral indices found among many prosimians or the high indices of apes. Most have a relatively short forearm, and most lack an opposable thumb. All have a tail of some sort, and in five genera the tail is a prehensile, fifth limb.

-from Primate Adaptation and Evolution by John G. Fleagle (Author)