Phalangeal Formula
The number of phalanges in animals is often expressed as a "phalangeal formula" that indicates the numbers of phalanges in digits, beginning from the innermost (medial or proximal).
Most land mammals including humans have a 2-3-3-3-3 formula in both the hands (or paws) and feet. Primitive reptiles typically had the formula 2-3-4-4-5, and this pattern, with some modification, remained in many later reptiles and in the mammal-like reptiles. The phalangeal formula in the flippers of cetaceans (marine mammals) is 2-12-8-1.
Read more about this topic: Phalanx Bone
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