idæ

from The Century Dictionary.

• In words of Greek origin, a suffix denoting the descendants of a person to whose name the suffix is attached, or a family or kindred of a particular origin: as, the Heraclidæ, Homeridæ, Eupatridæ, etc.
• Specifically In zoology, the regular termination of the names of families, suffixed to the stem of the name of the genus whence that of the family is derived, as Felidæ (from Felis), Laniidæ (from Lanius), Apodidæ (from Apus), etc.