Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         Biota
           "viruses"
             "DNA and RNA reverse transcribing viruses"
               "RNA Reverse Transcribing Viruses" [monotypic]
                 Family Retroviridae
                   1 Subfamily Orthoretrovirinae
                   2 Subfamily Spumaretrovirinae [monotypic]