As has already been stated, it can be difficult to nail down an "official"
pronunciation for a word in a language that hasn't been officially spoken
for centuries.  However, as has also been mentioned, there are specific
rules that govern pronunciation of words in classical Latin.  However, in
Latin, as in English, there are many exceptions to the rules; and, in modern
taxonomy we have taken a number of liberties with Latin which render it
anything but "classical", and make it impossible, or at least illogical, to
apply some of the rules.

The standard rule for pronouncing Latin nouns of more than three syllables
is to place the emphasis on the antepenultimate syllable.  In plain English,
that means third-from-last (ultimate = last; penultimate = next-to-last;
antepenultimate = before the next-to-last).  This works fine with a lot of
Latin or latinized words which have the form of regular nouns:

Crepidula = crep-ID-u-la
Cuspidaria = cusp-id-AR-i-a
Thatcheria = thatch-ER-i-a
Struthiolaria = struth-i-ol-AR-i-a
Gyrineum = gyr-IN-e-um
Inquisitor = In-QUIS-i-tor
Knefastia - kne-FAS-ti-a

However, many (very possibly a majority of) taxonomic names are not in the
form of regular Latin nouns.  Many generic names are built from Latin or
Greek (or other) roots by adding Latin suffixes and/or by combining two or
more short words to make one long word.

There are a number of common Latin suffixes which provide information about
the part of the word preceding.  For example, "-ellus" (and its female and
neuter counterparts "-ella" and "-ellum") is a diminutive ending.  It means
"small", in reference to whatever is denoted by the rest of the word.  This
suffix is ALWAYS pronounced with a strong emphasis on the first syllable -
EL-lus.  Therefore, any word ending with this suffix will have an emphasis
on the penultimate, not the antepenultimate syllable, regardless of its
total length:

vitellus - vit-EL-lus
Patella = pat-EL-la
Stomatella - stom-at-EL-la (not stom-AT-el-la)
Pyramidella = pyr-am-id-EL-la
capitanellus = cap-it-an-EL-lus
Haustellum - haust-EL-lum (not HAUST-el-lum)

The same is true for many other suffixes: -oideus; -ensis; -issimus;
-errimus; -idius; -arius; -osus; - atus; etc.  All such suffixes have a
strong emphasis on the first syllable, and that will normally be the
principle emphasis in the entire word, though some words may have a minor
emphasis on a preceding syllable.

Many taxonomic terms are created by combining smaller words.  For example,
the Fissurellid genus Amblychilepas is a combination of "amblys", meaning
blunt or obtuse; chilo, meaning a lip or rim; and lepas, meaning a limpet.
Therefore the name provides an apt description of a limpet with a thick,
obtuse lip.  The problem we run into in such cases is that folks who have
developed a "feel" for how Latin words most often sound will automatically
apply the antepenultimate rule in situations where it actually doesn't make
much sense.  Pronouncing the above genus "am-bly-CHI-le-pas" makes no sense.
It makes "lepas" sound like an add-on suffix, when actually it is the
substantive noun of the whole construction.  A LIMPET is what we are talking
about here.  The rest of the word is merely descriptive.  "lepas" is
pronounced "LE-pas", and that fact does not change as a result of enhancing
it with descriptive terms.  Therefore the genus is best pronounced
"am-bly-chi-LE-pas".

There are many generic names which include the word "stoma" ("STO-ma"),
meaning "mouth" - Calliostoma, Trigonostoma, Chrysotoma, Ceratostoma;
Chilostoma; Glyptostoma; and many specific names as well - erythrostoma,
luteostoma, iostoma, sarcostoma, macrostoma, brachystoma, and many others.
There is a strong tendency to apply the antepenultimate rule to such names -
tri-gon-OS-to-ma; call-i-OS-to-ma; cer-at-OS-to-ma.  In my opinion, doing so
subverts the entire meaning of the word.  "trigono-" means "triangular";
"callio-" means "beautiful";  "cerato-" means "horned"; "chryso-" means
golden.  When you join one of these words to "stoma", you get a meaningful
expression.  "tri-gon-o STO-ma" means "triangular mouth".
"tri-gon-OS-to-ma" doesn't really mean anything.

That brings me to the genus name which generated this whole thread -
Xenophora.  I must admit, I grew up saying "zee-NAW-for-a".  However, once I
realized that "phora" ("PHOR-a") means a carrier or bearer, and "xenos"
("XE-nos") means strangers or guests, I couldn't bring myself to say it that
way again.  "Xeno phora" literally means "bearer of strangers" or "carrier
of guests"!  What a great name for this wierd family of shells!  Both "xeno"
and "phora" have the emphasis on the first syllable, but since CARRIER is
the principle focus of the name, and the other part is descriptive, I now
pronounce the name with the main emphasis on the first syllable of "phora",
and minor emphasis on the first syllable of "xeno".

Ok how about Lithophaga . . . .  phaga means to eat . . . lithos means stone
. . .  I could go on forever . . . . but I won't.

Paul M.