What does 'Agg.' mean?

Thanks, JoC. That is a brilliant reference. In particular, I note: Including the subgenus - Nereis (Hediste) diversicolor. The subgenus name may be present or absent between the generic and specific name. If present it must be in parentheses () and with the first letter capitalised, such as Nereis (Hediste) diversicolor.
Explanation: A genus can have many species. Sometimes it contains so many species that taxonomists have created a subgenus to help keep track of the affinities within the genus. However, the subgenus may be totally omitted. It is an interpolation, not part of the species binomen.

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That’s the standard usage. I think I’ve also seen a name in brackets used to denote an alternative generic placement.