The ornamental design for fountain pen, as shown and described.
Description:
FIG. 1 is a top, left front perspective view of a fountain pen showing my new design with the cap member in an "in use" position and the writing tip exposed;
FIG. 2 is a top plan view thereof;
FIG. 3 is a rear elevational view with the cap member in a closed position;
FIG. 4 is a front elevational view of FIG. 3;
FIG. 5 is a right side elevational view of FIG. 3; and,
