Thermostat housing

[THREEFISHORANGE]

I removed the black plastic housing to reveal the thermostat. The thermostat is then housed in an off White plastic shroud. The thermostat I got from the shop is a round stat with rubber seal around the O.D. Without pulling it all to pieces, does the OE part come in that White plastic part, is it integral, or does it pull apart to accommodate the round one?