Abstract: The separator comprises an elongated, cylindrical, horizontal pressure vessel. An involute feed inlet introduces the drilling fluid into the top of the vessel chamber at its first end. The feed inlet is designed to reduce the flow velocity of the incoming feed stream and introduce it horizontally toward the first end head of the vessel. A weir, slanted away from the first end head, extends upwardly from a point close to the head's base. The curved head, vessel side wall and slanted weir form an inlet compartment of hopper-like configuration designed to funnel settling cuttings to a bottom outlet for removal with an auger pump. This design enables the cuttings to be removed while drilling progresses, without the build up of cuttings piles. The vessel also has a second weir at about its midpoint, for trapping fine solids in a second compartment. Riser outlets are provided in the second compartment for water removal and in the third compartment for oil removal. Gas is removed through an overhead outlet.