월세계(月世界, Epithelantha micromeris)
                                         단추 선인장이라 일컫는 북미 둥근선인장류의 월세계

 
  
                          월세계 (군생)                                                                 월세계 철화                    

    
                                          신월환(新月丸) Epithelantha micromeris ssp. polycephala

   
 오월환(烏月丸) Epithelantha micromeris ssp. unguispina                     Epithelantha micromeris ssp. (?)

  
                                         학란(鶴卵, 학의 알) Epithelantha micromeris var. longicaulis