Arecidae

Arecidae is a botanical name at the rank of subclass. Circumscription of the subclass will vary with the taxonomic system being used (there are many such systems); the only requirement being that it includes the family Arecaceae.
Arecidae in the Takhtajan system.
The Takhtajan system used this name for a subclass in the class Liliopsida (=monocotyledons).
Arecidae in the Cronquist system.
The Cronquist system (1981) used this name for a subclass in the class Liliopsida (=monocotyledons), with the circumscription:
APG II system.
The APG II system does not use formal botanical names above the rank of order; it assigns the plants included in this subclass by Cronquist in various taxa throughout the clade monocots. In other words, by today's standards Cronquist's subclass was not a "good" group.