Definition: "Skeletal element in the midline of the pelvic girdle that arises from somites 10-13. It is a rodlike caudal bony structure unique to anurans that lies within a compartment formed by the elongated, anteriorly oriented pelvic girdle. It is formed by the fusion of the neural arches of the 10th-13th vertebrae, and the posterior-most remnants of the hypochord. Timing of complete ossification varies between NF stages 62 and 66."