As nouns the difference between diatom and hypotheca

is that diatom is a group of minute unicellular algae having a siliceous covering of great delicacy, now categorized as class class: Diatomophyceae or division Bacillariophyta, formerly all included in the order order: Diatomaceae, now obsolete while hypotheca is the lower or posterior half of the theca of a thecate protist such as a diatom or dinoflagellate.