pachytheca

from The Century Dictionary.

• noun Minute hollow spheroidal bodies of brown color described by Hooker, 1853, from the Ludlow bone-bed of the English Upper Silurian. They resemble Sporangites from the Devonian of North America, and they appear to be spores of lycopods or algæ, though their vegetable nature has been doubted by Solms-Laubach.