Sentence:  Laurence M . Klauber put length at maturity at two <WSD>thirds</WSD> the ultimate length for some rattlesnakes , and Charles C . Carpenter 's data on Michigan garter and ribbon snakes ( Thamnophis ) show that the smallest gravid females are more than half as long as the biggest adults . 
Meaning of the word marked with <WSD> tags:  one of three equal parts of a divisible whole