How to change a circle into a square with d3.js

After appending 25 circles to the page, I run the following function:

var transitionPage = function () {
  startThePage();
  var height = $(document).height() - 20
    , width = $(document).width()
    ;

  d3.selectAll("circle")
    .transition().duration(2500)
      .style("fill", "steelblue")
      .attr("r", 15)
    .transition().duration(1000)
      .attr("cy", (height / 2))
    .each(function (d, i) {
      d3.select(this)
        .transition().duration(1000)
        .attr("cx", 30 + (i * width / 25));
    });
}

This works well and correctly lines them up horizontally along the middle of the page.

However, I couldn't figure out how to then transform each circle into a square or rectangle.

How should I approach this problem?