Monkey
Monkeys are a group of primates found in both the "New" and the "Old" World. The term "monkey" is actually a laypersons term for any one of the generally small-bodied and often distantly related primates that do not fall into the classification of prosimians or the generally larger bodied apes and humans. There are two basic types of monkeys - platyrrhines and catarrhines[1][2]. South American monkeys are known as platyrrhines and differ substantially from the catarrhines monkeys, having evolved largely in isolation[1][2].
Platyrrhine monkeys
The name platyrrhine is derived as a descriptive term of the monkeys faces, since South American monkeys have broad, flat noses. All platyrrhine monkeys are small - the largest is only around 10kg[1]. They also exhibit many primitive dental and skeletal features including having three pre-molars in each side of their mandible and maxilla - thus they have a total of twelve premolars [2].
All platyrrhines have a tail of some sort and five genera have prehensile tails[1]. The most well known of the prehensile-tailed monkeys is probably the spider monkey. Interestingly, the spider monkey has no thumb[1].
Catarrhine monkeys
Catarrhines monkeys are "Old World Monkeys" and all fall under the Superfamily Cercopithecidae[2]. They have narrow noses, eight pre-molars and none have prehensile tails[1]. There are two Subfamilies within the Superfamily - Cercopithecinae and Colobinae[2]. There are approximately sixty species of cercopithicus monkey and about forty species of colobus monkey.