The ancient Greek word for a hair is thrix (nominative, singular). The t is softened by the h, a phenomenon called aspiration; the x at the end is hard. In a syllabe only a single aspiration is allowed. The genitive singular of thrix is trichos: the aspiration has shifted to the x, so the t has lost it. (I cannot explain why, I am a botanist, not a linguist; this is what I remember from school) The names of higher taxa are derived from the root of a word (which is obtained by cutting of the ending of the genitive from, in this case trichos ->trich) followed by the appropriate suffix, -idae for family. Result Callitrichidae, which by the way means with beautiful hairs, not Callithrichidae. Similarly the filamentous green alga Ulothrix belongs to the Order Ulotrichales. I hope that I have been helpful, Albertine