New Zealand alone has at least 1000 KNOWN Recent undescribed marine
species (and at least a similar number of known undescribed fossil
spp), with many more turning up all the time. I have literally dozens
of undescribed species, both fossil & Recent, in my collection.
Remember that 99% of all molluscan spp are micros (say under 1cm)...
Eventually Ross' guesstimate may well prove rather conservative!