Plural form of nouns ending in is?

Some nouns ending in -is are made plural by changing the ending to -es; examples:

  • axis - axes
  • basis - bases
  • crisis - crises
  • diagnosis - diagnoses
  • ellipsis - ellipses

Some nouns ending in -is are regular plurals adding the ending -es to the end of the word; examples:

  • iris - irises
  • mantis - mantises
  • metropolis - metropolises
  • pelvis - pelvises
  • trellis - trellises