Sentence:  Laurence M . Klauber put length at maturity at two thirds the ultimate length for some <WSD>rattlesnakes</WSD> , 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:  pit viper with horny segments at the end of the tail that rattle when shaken