Sentence:  In a valve of the type herein described, the combination of a valve casing provided with a partition dividing its interior into an inlet chamber, an outlet chamber and a piston chamber, the piston chamber having a lower portion of greater area than its upper portion, the said partition having a passage connecting the inlet and outlet chambers, the <WSD>undermargin</WSD> of said passage being formed into a valve seat, a restricted passage in the aforesaid partition connecting the inlet chamber with the lower end of the piston chamber, a piston valve disposed in said piston chamber and having upper and lower portions corresponding in diameter to that of the upper and lower portions of the piston chamber and moving therein, .... 
Meaning of the word marked with <WSD> tags:  The lower rim or margin of something.