Single-Celled Alga
This page is the CodyCross answer for the clue below from Prehistory, Group 1246, Puzzle 1 (Extinction).
Question is: Single-Celled Alga
and the answer is: DIATOM
Word Meaning:
DIATOM — Any of a group of minute unicellular algae having a siliceous covering of great delicacy, now categorized as class Diatomophyceae or division Bacillariophyta.
Definitions from Wiktionary.