In most specimens of Necturus, the nineteenth vertebra serves as the sacrum; the iliac processes of the pelvic girdle articulate with it. In a number of specimens, Vertebra XX or (less frequently) Vertebra XVIII serves as the sacrum. But not always do the two iliac processes join the same vertebra. The iliac process of the left side may join the XVIII, XIX, or XX vertebra. The converse may also occur.
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Harris, Joseph P. Jr.
"Variations In The Vertebrae And Pelvis Of Necturus,"
Field and Laboratory: Vol. 18
Available at: https://scholar.smu.edu/fieldandlab/vol18/iss2/9