One last single simile in this little series. Comparing anything with a rose is a serious poetic cliché, and that was true even back when Shelley was writing. Still, we think he does a pretty good job. In this case, he compares the sound of the invisible skylark to the smell that comes from a hidden rose.
Line 51: The more time you spend with this poem, the more little patterns you see. In the first simile, we covered (the maiden) the speaker talked about her song, so that was about the sense of hearing. In the comparison to the glow-worm, it was about the bug's light, so that was about the sense of sight. Finally, this comparison to the rose is about fragrance, and the sense of smell. That play with both repetition and difference is part of what makes Shelley's work so effective.