The spelling of the word "vitelluses" can be confusing due to its irregular plural form. The word refers to the yolk sacs of eggs, particularly those of fish or insects. Phonetically, it is spelled /vɪˈtɛləsiːz/, with stress on the second syllable. The plural is formed by adding -es to the singular "vitellus". This is an example of how English has many irregular plural forms that can be difficult to navigate, even for native speakers.