Since I am a little bit obsessed with marginalia at present I thought I’d reproduce the list, make some additions, and—at some point in the future—add images of examples I encounter in works by Haywood.
arrow: a mark like an arrow, or arrow-head, used as a pointer
asterisk: frequently drawn as a small x-cross with a dot in each angle
asterism: a group of three asterisks placed thus (***) to direct attention to a particular passage
brace: a sign ( } or ] or > ), but may take more improvisational shapes) used in writing or printing, chiefly for the purpose of uniting together two or more lines, words, staves of music, etc.
caret: an inverted-v shaped mark placed in writing below the line, to indicate that something (written above or in the margin) has been omitted in that place
cross: two bars or lines (horizontal and vertical) crossing each other, used as a sign, ornament, etc.; mark or sign of small size used to mark a passage in a book, etc.
[dagger: †; see cross]
dash: a horizontal stroke (usually short and straight)
dot: a minute roundish mark
double oblique: two parallel slashes ( || ) or diagonal strokes ( // )
double triangle: two adjoining triangles sharing a horizontal base line
flower: the representation of a flower of more than three or four petals (which would be trefoils and quatrefoils; see below)
gnomic pointing: double inverted commas used in the meadieval and early-modern period to draw attention to proverbs and sententiae
label: a slip of paper, cardboard, metal, etc. attached or intended to be attached
line: a horizontal line, longer than a dash (and generally serving a different purpose)
manicule: hand or fist with pointing finger
marginal commas: single or double commas, sometimes inverted, used to mark a line or lines of text. Alexander Pope used a system of marginal commas and asterisks in his Chaucer and Shakespeare to indicate “some of the most shining passages.”
mathematical formulas: use only for complex numeric equations or arithmetical problems; transcribe simple numeric or mathematical annotations in full
n.b. or N.B.: abbreviation for nota bene, or "note well"
O: the letter considered with regard to its shape
oblique: a slash or diagonal stroke
quatrefoil: compound leaf or flower containing four, usually rounded, leaflets or petals radiating from a common centre
[quotation marks: see gnomic pointing, marginal commas and running quotes]
running quotes: double inverted commas used to indicate a quotation and, therefore (perhaps), something quote-worthy
scribble: a piece of random or casual doodling or drawing of unclear textual purpose, including pen trials made by writers to test a freshly-trimmed pen or a writing style
stroke: a vertical stroke (usually short and straight: | )
trefoil: a leaf, such as a clover, comprising three rounded sections
triangle: a rectilineal figure having three angles and three sides
X: the letter considered with regard to its shape
[UPDATED 19 July 2012]