Sentence:  Also it is advisable to distinguish this ( ˘ ˘ ) — ˘ ˘ — rhythm, where the princeps was probably shorter in duration than the <WSD>biceps</WSD> (as in the dactylic hexameter), from true (marching) anapaests, in which they were equal. 
Meaning of the word marked with <WSD> tags:  A point in a metrical pattern that can be filled either with one long syllable (a longum) or two short syllables (two brevia)