The strokes on plate 20 are so well defined that it hardly seems
necessary to describe them. For 1 a short piece of chalk is used, the
side of the chalk giving the width of the line. At 2 the stroke is
similar, the accent being at one end of the chalk. At 3 the stroke is
slightly curving, the chalk being placed vertically, and the accent
being upon the upper end of the chalk. No. 4 is drawn by placing the
chalk vertically upon the board, drawing it downward very quickly
and twisting it to the horizontal position. Apply these strokes in
sketching the lobster. First use stroke 3, then touches like 2;
afterward strokes 1 and 4 for details.
No. 5 indicates the strokes first used in sketching the shells. In
drawing the outside of the nautilus, use a long piece of chalk and with
a curving stroke accented with the end, form the outline. With strokes
similar to those at 2, sketch the light streaks in the shell, and add
dark details with charcoal. Use the tip of the finger in softening the
tones here and there.
In drawing the section, sketch first the spiral curve, then the blended
strokes connecting the outer with the inner curves of the spiral, and
add charcoal for shadows.
The oyster shell is drawn by the use of the lower stroke at No. 5.
Make the stroke, accenting a little at the left end of the chalk; then
reverse the stroke, accenting with the right end, and add details with
chalk and charcoal.
[Illustration: Plate 21]