Fellow WIPS member Shellie Luallin, who is expert in 3D imaging of fossils
https://sketchfab.com/Paleogirl
recently imaged a presumed Pennsylvanian blastoid of mine from Cherokee County, OK and generously made it available as a free download:
https://sketchfab.com/3d-models/pentremites-rusticus-e729f54539014770b0128b000fca841b
Note the very pronounced interambulacral areas (deltoids) where the hydrospires are developed.
Katz (1978)
https://www.jstor.org/stable/1303971?seq=1
lumped exisiting variants into one species P. rusticus based on hydrospire similarity
Given that this distinctive morphologic end member is stratigraphically restricted and present a population, perhaps this conclusion should be revisited.