Click a Dropdowm Menu Button using Watin

I am testing a website using Watin in the page there is dropdown menu button like [Download|V] if I find that button and click it clicks the entire button and selects the first Option in the menu but I want to click the V of the button to show the dropdown menu(div) and select the option from it the code I had tried to click the button

ff.Button(Find.By("aria-describedby", "x-auto-456")).FireEvent("onmouseover");
ff.Button(Find.By("aria-describedby", "x-auto-456")).FireEvent(" Onmousedown");
ff.Div(Find.ById("x-auto-457")).Click();

the Button HTML

<button class="x-btn-text" style="position: relative; width: 16px;" type="button" tabindex="0" aria-describedby="x-auto-456">