need horizontal scrollbar for a select box which has fixed width

i want to add a horizontal scroll bar in select box which has fixed width.i tried using overflow:auto; it is not working properly is there any other solution.
i tried like this
<div style="overflow:auto;width:300px;" align="left">
<select name="selectedTree" id="selectedTree" multiple size="16" align="left">
</select>
</div>