StopPropagation() with SVG element and G

I created an SVG element with an .on("click") behavior and appended g elements with .on("click") and thought that I could use d3.event.stopPropagation() to keep the SVG click event from firing with the g click event. Instead, both events continue to fire. So I must be placing stopPropagation in the wrong place.

svg = d3.select("#viz").append("svg:svg")
    .attr("width", 800)
    .attr("height", 800)
    .on("mousedown", mousedown);

sites = svg.selectAll("g.sites")
    .data(json.features)
    .enter()
    .append("svg:g")
    .on("click", siteClick)
    ;

sites.append("svg:circle")                
    .attr('r', 5)
    .attr("class", "sites")
    ;

function mousedown() {
    console.log("mouseDown");
}

function siteClick(d, i) {
    d3.event.stopPropagation();
    console.log("siteClick");
}