No significant correlation between number and length of caudal vertebrae in a laboratory intercross. Each point represents the number of caudal vertebrae and the length of the longest caudal vertebra measured from a radiograph of a parental type (A, P. m. nubiterrae (green), P. m. bairdii (tan), n = 12 of each subspecies), or first-generation F1 (B, n = 10) or second-generation F2 (C, n = 96) nubiterrae x bairdii hybrids. Ninety-six F2 individuals allow 80% power to detect a correlation of r > 0.25. [Color figure can be viewed at wileyonlinelibrary.com]