What is the correct plural form of thesis?

Read the excerpt from "the most dangerous game." again rainsford lifted the heavy knocker, and let it fall. the door opened then, opened as suddenly as if it were on a spring, and rainsford stood blinking in the river of glaring gold light that poured out. the first thing rainsford's eyes discerned was the largest man rainsford had ever seen—a gigantic creature, solidly made and black bearded to the waist. in his hand the man held a long-barreled revolver, and he was pointing it straight at rainsford's heart. the details of this excerpt show that rainsford sees ivan as a(n) individual.