referring to the drawings , an embodiment of the marking pen of the present invention is shown comprised of sheath 10 having writing member 11 disposed therein , and sealing member 13 disposed within said sheath generally below said writing member . an actuating spring 12 is shown as a conventional feature of a retractable pen . the sheath , having an elongated hollow configuration , is fabricated of a reasonably rigid material , and serves as the outer body of the pen . the sheath has an upper portion 14 and a lower portion 15 terminating in open front end 16 centered upon the longitudinal axis 17 of the pen . the inner wall 18 of the sheath is of a generally circular cylindrical configuration . in alternative embodiments , the sheath may be of shorter length which does not completely confine the writing member . such shorter sheaths are particularly useful in wide diameter markers . the writing member , which is coaxially centered within the sheath , is comprised of an elongated rear portion 19 of relatively large diameter which serves as a confining reservoir for marking fluid , and an elongated cylindrical front portion 20 of relatively small diameter which serves as a marking fluid conduit and terminates in writing tip 21 which may be a ball or felt . actuating means of conventional design ( not shown ) will generally be associated with the upper portion of the sheath for causing movement of the writing member between a forwardly held writing position and a rearwardly held non - writing position . actuating spring 12 is interactive between a pressing shoulder 22 inwardly directed from the lower portion of the sheath , and the rear portion 19 of the writing member . in such manner of disposition , the actuating spring serves to drive the writing member to its retracted non - writing position and to hold it there . in alternative embodiments , particularly those with sheaths of shorter length , the actuating spring may not be utilized . in such embodiments , a snap fit may be configured between portions of the writing member and the sheath to hold the writing member in its writing and non - writing positions . equivalent holding means may , however , be utilized . sealing member 13 is of monolithic construction , having been fabricated of an elastomer by molding or other shaping methods . the sealing member is comprised of upper portion 23 having cylindrical bore 24 centered therein and centered upon the pen axis . the diameter of bore 24 is just slightly larger than the diameter of front portion 20 of the writing member . upper portion 23 is bounded from above by top surface 25 radially disposed to said pen axis . alternative embodiments of the sealing member may be of non - monolithic construction , fabricated of two or more specially interactive components . the lower portion 26 of the illustrated sealing member has two pendant segments 27 having contoured inner and outer surfaces 28 and 29 , respectively . the segments are adapted to move between a divergent state shown in fig1 and 3 , and a converged state shown in fig2 and 4 . in the converged state , the segments are in their closest approach to the pen axis , and their inner surfaces mate to form a chamber 30 whose upper extremity communicates with said bore . in other embodiments , three or more such segments may be utilized . in still other embodiments the sealing member , by virtue of its elasticity , may be configured to allow the segments to move to their divergent state without lateral separation , thereby reducing the size of the inner surfaces which must mate to form a chamber . the intermediate portion 31 of the sealing member has recesses 32 which permit the segments to move in hinge - like manner with respect to upper portion 23 . in alternative embodiments , movement of the segments may be accomplished without such recesses . pressing shoulder 22 is positioned and configured to engage the outer surfaces of the segments . it is to be noted that the upper extremity of said outer surfaces has an outwardly directed abutment shoulder 33 . the lower extremity has an outwardly tapered region 34 . an inwardly depressed trough - like zone 35 is disposed between shoulder 33 and tapered region 34 . by virtue of such configuration of the outer surfaces of the segments , and their interaction with pressing shoulder 22 , the segments are directed to their divergent state when the sealing member is urged downwardly , and are directed to their coverged state upon upward movement of the sealing member . in alternative embodiments , two pressing shoulders may be positioned and configured to engage outer surfaces of the segments . in such embodiments , outer surfaces of the segments may be specially contoured to interact with said pressing shoulders such that one pressing shoulder directs the segments to their divergent state when the sealing member is urged downwardly and one pressing shoulder directs the segments to their converged state upon upward movement of the sealing member . a restraining hip 38 protruding outwardly from tapered region 34 is positioned and configured to engage pressing shoulder 22 to prevent excessive upward movement , and consequent dislodging , of the sealing member . coil spring 36 , compressively interactive between rear portion 19 and top surface 25 , is disposed coaxially within actuating spring 12 . coil spring 36 thereby serves the functions of : ( a ) driving the sealing member downward and ( b ) holding the sealing member in its downwardly urged position . an activating shoulder 37 is associated with the front portion of the writing member , and is adapted to abut against the interior surface of the sealing member during upward , retracting motion . said activating shoulder thereby serves the functions of : ( a ) driving the sealing member upward , ( b ) holding the sealing member in its upwardly urged position , and ( c ) forming a seal against bore 24 to cause chamber 30 to become an air - tight enclosure for writing tip 21 . in other embodiments , the bore and the interior surface of the sealing member may be specially shaped to seat said activating shoulder . in other such embodiments , said activating shoulder may have the additional function of assisting the coil spring in driving the sealing member downward . while particular examples of the present invention have been shown and described , it is apparent that changes and modifications may be made therein without departing from the invention in its broadest aspects . the aim of the appended claims , therefore , is to cover all such changes and modifications as fall within the true spirit and scope of the invention .