xkcd.WTF!?

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Dental Formulas

I mean, half of these are undefined. And your multiplication dots are too low; they look like decimal points.

Explanation

A dental formula specifies the number of teeth of each type on each side of the jaw for a given species, with dots separating the numbers. There are two rows, representing the upper and lower jaw, separated by a horizontal line. The number of incisors is indicated first, canines second, premolars third, and finally molars, so the formula in the comic would represent 3 incisors, 1 canine, 3 premolars, and 1 molar on each side of the upper jaw, and equal numbers in the lower jaw except only 2 premolars. This is the dental formula for the cat family. The adult human dental formula is 2.1.2.3 for both the upper and lower jaw.

Cueball is (wrongly) treating a dental formula as an arithmetic expression, with the line indicating division and the dots indicating multiplication. In the title text his statement that half the formulae are undefined refers to animals that lack one of the four types of teeth in the lower jaw, leading to a zero in the "denominator" of the dental formula and an undefined division expression. He also notes that the "dots are too low", as in fact the dots in a dental formula are period characters and aren't meant to imply multiplication, which uses middle dot characters.

The word 'mammologist' is an alternate spelling of 'mammalogist', for one who studies mammals. Or, in some cases, specifically studying the mammaries (i.e. breasts) which mark out mammals in general. The specific study of teeth might be termed 'odontology', so we should assume that the experts who Cueball is referencing are not specifically tooth-focused, merely using this particular specialism to help with their own particular, arguably far wider, brief that is not so entirely fixated solely upon any particular body parts.