Classes (6)
I believe the answer is:
genera
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">classes</span>' is the definition.<br/ ><span class="explanation-format__explanation">(more than one genus - a class or category)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for genera that I've seen before include "Forms" , "Categories making up a family" , "Blood vessel" , "Biological groups" , "similar groups" .)