1. A pitless well adapter for attachment to a well casing having an opening therein spaced below the upper end of the casing, comprising: a mounting member having a front face and a rear face and adapted to be positioned interiorly of the casing against the inner surface thereof, said mounting member having an opening therein and having an annular lip integral with said front face and projecting forwardly therefrom, said mounting member having a pair of vertically extending elongate spaced apart guide elements on the rear face thereof and defining a pair of downwardly tapering guideways on opposite sides of said opening, a cylindrical nipple integral with said mounting member and projecting forwardly from the front face thereof through the opening in the casing, said nIpple communicating with the opening in the mounting member and having an externally and internally threaded outer end portion, the external diameter of said nipple being only slightly less than the diameter of the opening in the casing to define an annular space therebetween, an interconnecting passage extending through the cylindrical wall of the nipple and intercommunicating the interior of the nipple with the annular space, a coupling member having a passage therethrough the being adapted to be connected in communicating relation with the well pipe, a pair of vertically extending spaced apart, elongate downwardly tapering guide elements on said coupling member engagable with said guideways on said mounting member to urge said coupling member into fluid sealing relation with the rear face of the mounting member and to intercommunicate the passage of the coupling member with the opening in the mounting member, an interior annular compressable sealing element positioned around and engaging said nipple and being engaged by said lip of said mounting member, the interior sealing element adapted to be interposed between the front face of the mounting member and the inner surface of the casing to form a seal thereat, an exterior sealing element positioned around and engaging the nipple and adapted to engage the exterior surface of the casing adjacent the opening therein, said sealing elements cooperating with said nipple and said casing to define an annular pressure passage located between the sealing elements and exteriorly of the nipple, an annular clamping member corresponding generally in size to the size of said exterior sealing element and engaging the latter, and an internally threaded clamping nut threadedly engaging the exterior threads on the nipple and urging said clamping member into clamping relation with the exterior sealing element and clamping the mounting member on the casing.