Dental Comb: In lemurs and lorises, the lower incisors and canines project away from the head, parallel to the jaw, to form the dental comb.  It is used for grooming the coat and applying scent to the body, as well as for gouging trees to get sap.

Intermembral Index (IMI): The ratio of forelimb length to hindlimb length, as a measure of limb proportions.  If an animal's intermembral index is around 1, that animal is probably a quadruped.  If it is less than 1, the animal probably gets around by leaping or bipedalism.  If it is greater than 1, the animal probably uses suspension or brachiation.

Jacobsen's Organ: A part of the brain which detects chemicals in fluids, used by primates for recognizing and interpreting scent markings.  It is well developed in lemurs and lorises, and still used by platyrrhines, but it is vestigial or absent in catarrhines and apes.

Kay's Threshold: The observation that animals weighing under 500 grams can not sustain themselves on leaves, while animals weighing over 500 grams can not sustain themselves on insects.

Scent Markings:  Scent markings are extremely important for communication between primates, especially those primates which forage solitarily, because both animals need not be present for the message to be passed.  They are detectable from a distance and are not washed away by rain.

Secondary Characteristics: An animal is said to be secondarily nocturnal when in its evolutionary history it was once nocturnal, then became diurnal, and now is nocturnal again.  Likewise, an animal is said to have secondary claws if its ancestors once had claws and then developed nails, and now the living animal has re-evolved claws from nails, as in the case of marmosets and tamarins.

Tapetum Lucidum: A reflective layer behind the retina, found in many nocturnal animals, which helps animals see better in low light.  It works by passing the light back through the rods and cones a second time, thus increasing the response of the eye.  This is what you are seeing when you see a cat's eyes shine at night.  The tapetum is found in lemurs and lorises, but not in tarsiers, which leads us to suspect that tarsiers may be secondarily nocturnal, having lost the tapetum when they were diurnal.  To compensate for this, tarsiers have enormous eyes and all of the sensory cells in their eyes are rods, which sense light, without any cones, which sense color.