1 literature reference for Cylindropuntia echinocarpa (Engelm. & J.M. Bigelow) F.M. Knuth:

Author: Flora of North America Editorial Committee, eds.
Year: 2003
Title:
Source: Flora of North America North of Mexico, vol. 4: Magnoliophyta: Caryophyllidae, part. 1, The volumes are also reproduced online (http://www.fna.org/)