There is no hard-and-fast rule about the use of "cf." and other
abbreviations. However, the following usages are very widespread in
biologic circles, and are required by some paleontologic journals:
 
Monfiloconchus pauli: The identification is certain.
Monfiloconchus cf. pauli (or cf. M. pauli): The identification is uncertain
because the material is fragmentary or otherwise flawed.
Monfiloconchus aff. pauli (or aff. M. pauli): The material is good enough
to identify, but may belong to an unnamed species related to M. pauli.
Monfiloconchus pauli?: Definitely Monfiloconchus, but may not be pauli.
Monfiloconchus? pauli: Definitely pauli, but this species may not belong to
Monfiloconchus (for instance, the taxonomy of this genus may need
revision.)
?Monfiloconchus pauli: Both genus and species are in doubt.
 
You will occasionally see other abbreviations, especially in the older and
more technical literature. Schenk and others (1948, Procedure in Taxonomy,
p. 27-29) list three pages of Latin words and abbreviations, including the
following, which I have modified slightly:
 
aff. = affinis = having affinity with but not identical with
auct. = auctorum = of authors [that is, of various authors]
cf. = confer = to be compared to
emend. = emendatio = emended
f. = forma = form
fide = trusting [the written word of]
gen. et sp. nov. = genus [novum] et species nova = new genus and species
ibid. = ibidem = the same [reference]
incertae sedis = of uncertain position
in litt. = in litteris = in correspondence
lapsus calami = a slip of the pen
loc. cit. = loco citato = [in the] place cited [to the page]
mihi = [belonging] to me (i.e., my new species)
nob. = nobis = [belonging] to us
nom. nov. = nomen novum = new name
nom. nud. = nomen nudum = nude name (new name given without an illustration
or description)
non = not
nec = nor
non vidi = I have not seen [it]
non viso = not seen
nov. = novum = new
olim = formerly
op. cit. = opere citato = [in the] work cited
partim = part
passim = here and there
pro parte = in part
q. v. = quod vide = which see
s.l. = sensu lato = in a broad sense
s.s. = sensu stricto = in the strict sense
sic = thus; [this is] so (that is, exactly as shown)
sp. = species = species (singular)
spp. = species = species (plural)
sp. nov. = species nova = new species
subgen. = subgenus = subgenus
supra cit. = supra citato = cited above
teste = according to [the spoken word of]
vide = see
 
Schenk, Edward T., McMasters, John H., Keen, Myra A., and Muller, Siemon
William, 1948, Procedure in taxonomy (2nd edition): Stanford, California,
Stanford University Press, vii + 93 p.
 
Also, an exclamation point (!) is occasionally used to indicate that the
author has seen a particular specimen, not just read about it.
 
Andrew K. Rindsberg
Geological Survey of Alabama