peri- + plastronal
periplastronal (not comparable)
- Surrounding a plastron
2016 February 6, “Ecophenotypic Variation and Developmental Instability in the Late Cretaceous Echinoid Micraster brevis (Irregularia; Spatangoida)”, in PLOS ONE[1], →DOI:The landmark configuration included the shape of the periplastronal ambulacrals and paired petals.