This is the opisthosoma (posterior part of the body) from a mangrove horseshoe crab (*Carcinoscorpius rotundicauda*)
Accession number: 329.13A
More info on this species here: https://en.wikipedia.org/wiki/Mangrove_horseshoe_crab
Captured with Artec Spider 3D Scanner
https://www.artec3d.com/portable-3d-scanners/artec-spider-v2