The ornamental design for a lower part of a helmet, as shown and described.
FIG. 1 is a left side elevation of the lower part of a helmet, showing my new design;
FIG. 2 is a right side elevation thereof;
FIG. 3 is a front perspective view thereof;
FIG. 4 is a rear perspective view thereof;
FIG. 5 is a front elevation view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a rear elevation view thereof; and,
FIG. 8 is a top plan view thereof.
The even dashed broken lines represent environmental structure that forms no part of the claimed design. The dot-dash broken lines represent boundary lines.