Chiroptera is a rare, extreme form of syndactyly. The fingers on one or both hands are extremely lengthened, and joined by a thick, dark membrane, causing the hand to resemble a bat wing. Often it is combined with polydactyly or hypodactyly, and the two hands are not always symmetrical. The condition is present in fewer than 1 in 100 million live births.

Although cases have been reported in medical literature throughout history, only two have been confirmed in modern times. Both are members of a Texan Mennonite family which believes they are God's chosen people and were elected to repopulate the Earth. After many generations of inbreeding, Rebekah X and Zachariah X were born within two years of each other, each exhibiting the most extreme cases of chiroptera ever reported. In both of those instances, the condition was comorbid with Down's syndrome.

[...no, really]