The Collaborative International Dictionary of English v.0.48:

Subclass \Sub"class`\, n.
   One of the natural groups, more important than an order, into
   which some classes are divided; as, the angiospermous
   subclass of exogens.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "subclass":
   biotype, branch, class, family, genotype, genus, kingdom, order,
   phylum, section, series, species, subfamily, subgenus, subkingdom,
   suborder, subspecies, subtribe, superclass, superfamily,
   superorder, superspecies, tribe, variety