The ornamental design for a fountainpen, as shown and described.
FIG. 1 is a side perspective view of a fountain pen showing my new design;
FIG. 2 is a front elevational view;
FIG. 3 is a right side elevational view, the left side being a mirror image
thereof;
FIG. 4 is a rear elevational view;
FIG. 5 is a top plan view;
FIG. 6 is a bottom plan view;
FIG. 7 is a front perspective view with the top removed; and
FIG. 8 is a rear perspective view of the fountain pen with a front
perspective view of the top, which is placed on the end remote from the
writing point.