So I read through the thread How to get Floating DIVs inside fixed-width DIV to continue horizontally? and used pd.'s great answer to build a fixed-size DIV to contain inline elements that would scroll horizontally.
It worked, it did exactly as I wanted... except the horizontal scrollbar doesn't actually work. If I make the DIV's height 140px so that there's also a vertical scrollbar, then it works. But when I make the diff tall enough to only have a horizontal scrollbar, it doesn't work! It's racking my brain. Here's the code:
<div style="height: 180px; width: 694px; margin: 0; padding: 0; overflow-x: auto; white-space: nowrap;">
<span style="width: 160px; height: 120px; background: rgba(255,255, 255, 0.9); display: inline-block; margin: 5px 0 5px 0;"></span>
<span style="width: 160px; height: 120px; background: rgba(255,255, 255, 0.9); display: inline-block; margin: 5px 0 5px 13px;"></span>
<span style="width: 160px; height: 120px; background: rgba(255,255, 255, 0.9); display: inline-block; margin: 5px 0 5px 13px;"></span>
<span style="width: 160px; height: 120px; background: rgba(255,255, 255, 0.9); display: inline-block; margin: 5px 0 5px 13px;"></span>
<span style="width: 160px; height: 120px; background: rgba(255,255, 255, 0.9); display: inline-block; margin: 5px 0 5px 13px;"></span>
</div>
If you'd like to look at the page directly, http://12for12boston.tumblr.com, PW: 12412.
Thanks guys!