there is still some discussion on the correct name:
Micromelo undata, Micromelo undatus or Micromelo undatum
Dr Richard Willan writes at the  Sea Slug Forum 
"The name Micromelo is feminine and, because the specific name is an adjective meaning wavy, it must agree with the gender of the genus. So the correct name is Micromelo undata (Bruguiere, 1792)."

according to WoRMS - World Register of Marine Species
http://www.marinespecies.org/index.php
is the accepted name Micromelo undatus (Bruguière, 1792) [Bulla]

Gosliner, Terrence M., David W. Behrens & Ángel Valdés. 2008. Indo-Pacific Nudibranchs and Sea Slugs. Sea Challengers Natural History Books. Gig Harbor, Washington. 426pp. 
use the name Micromelo undatum (Bruguière, 1792) [Bulla]

formerly:
Class: GASTROPODA   Cuvier, 1797 
Subclass: OPISTHOBRANCHIA Milne-Edwards, 1848 
Order: CEPHALASPIDEA   Fischer, P., 1883
Superfamily: Acteonoidea  d'Orbigny, 1843 

according to Bouchet & Rocroi (2005)
is the superfamily: Acteonoidea no more in the Order Cephalaspidea
now it is with the superfamilies
Architectonicoidea
Glacidorboidea
Mathildoidea
Nerineoidea (fossil)
Omalogyroidea
Pyramidelloidea
Ringiculoidea
Rissoelloidea
Siphonarioidea
in the new informal group Lower Heterobranchia (= Allogastropoda)
have a look at
Bouchet P., Rocroi J.-P., Frýda J., Hausdorf B., Ponder W., Valdés Á. & Warén A. (2005). 
"Classification and nomenclator of gastropod families". 
Malacologia: International Journal of Malacology (Hackenheim, Germany: ConchBooks) 47 (1-2): 1–397.
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