Cannot get horizontally-listed ul to overflow horizontally (and scroll)

I'm trying to get an unordered list to be listed out horizontally (accomplished with float:left), but it refuses to overflow horizontally. Instead, it automatically overflows in the next line (furthermore, even with overflow-y:none, it creates a vertical scrollbar automatically. Any ideas?

<style type="text/css">
    ul {
        height:15px;
        width:400px;
        overflow-y:none;
        overflow-x:auto;
    }

    li {
        float:left;
    }

</style>

<body>
    <div>
        <ul id="someList">
            <li>element 1</li>
            <li>element 2</li>
            <li>element 3</li>
            <li>element 4</li>
            <li>element 5</li>
            <li>element 6</li>
            <li>element 7</li>
        </ul>
    </div>
</body>